Assess fetal growth and understand your baby's birth weight relative to gestational age.
Enter the baby's gestational age in completed weeks (e.g., 39 for 39 weeks).
Enter the baby's actual birth weight in grams (g).
Male
Female
Select the baby's sex.
Your Birth Weight Z-Score Results
—
Mean Weight
— g
Standard Deviation
— g
Percentile Rank
—
Formula Used: The Z-score is calculated as (Observed Weight – Mean Weight) / Standard Deviation. The percentile rank is then derived from the Z-score using standard normal distribution tables or approximations.
What is a Birth Weight Z-Score?
The birth weight Z-score is a statistical measure used to evaluate a newborn's weight in relation to the expected weight for their gestational age and sex. In simpler terms, it tells us how many standard deviations away from the average weight a baby is for their specific stage of development. A Z-score of 0 indicates the baby's weight is exactly average for their gestational age. Positive Z-scores mean the baby is heavier than average, while negative Z-scores indicate a weight below average.
This metric is crucial for healthcare professionals to identify potential growth issues, such as small for gestational age (SGA) or large for gestational age (LGA) infants. Early identification allows for timely intervention and appropriate management, which can significantly impact a child's long-term health outcomes. It's important to understand that this is a statistical tool and should be interpreted by a qualified medical professional within the broader context of the baby's health and development.
Who Should Use It?
Primarily, obstetricians, neonatologists, pediatricians, midwives, and other maternal-fetal health specialists use birth weight Z-scores. Parents may also be interested in understanding their baby's growth trajectory as explained by their healthcare provider. It helps in assessing the effectiveness of prenatal care and identifying infants who might require closer monitoring or specific interventions after birth.
Common Misconceptions
A Z-score is a diagnosis: A Z-score is an indicator, not a diagnosis in itself. It flags a potential concern that requires further medical evaluation.
All low/high Z-scores are bad: While extreme Z-scores warrant attention, some babies naturally fall outside the "average" range without underlying problems. Genetics and parental size also play a role.
Z-scores are fixed: The Z-score is specific to birth weight and gestational age. As the baby grows, their growth pattern is tracked using different growth charts and percentiles.
Birth Weight Z-Score Formula and Mathematical Explanation
The calculation of a birth weight Z-score involves comparing an infant's actual weight to the mean (average) weight and standard deviation for babies of the same gestational age and sex. Specialized growth charts, like those from the World Health Organization (WHO) or the Centers for Disease Control and Prevention (CDC), provide these reference values.
The Core Formula
The Z-score is calculated using the following formula:
Z = (W – M) / SD
Where:
Z is the Z-score.
W is the infant's observed birth weight.
M is the mean (average) birth weight for the specific gestational age and sex.
SD is the standard deviation of birth weight for the specific gestational age and sex.
Derivation and Interpretation
To use this formula, you first need to find the appropriate mean (M) and standard deviation (SD) values. These values are typically obtained from published growth charts or statistical databases that are specific to the population and the reference standards being used (e.g., WHO, CDC, or local standards). These charts are often stratified by gestational age (in weeks) and sex.
Once the Z-score (Z) is calculated, it can be interpreted:
Z = 0: The baby's weight is exactly the average for their gestational age and sex.
Z > 0: The baby's weight is above average. A Z-score of +1.96 or higher is often considered significantly high.
Z < 0: The baby's weight is below average. A Z-score of -1.96 or lower is often considered significantly low.
The Z-score can also be used to estimate the baby's percentile rank. The percentile rank indicates the percentage of babies of the same gestational age and sex who weigh less than the observed baby. This is typically found by looking up the Z-score in a standard normal distribution table (Z-table) or using statistical software/approximations.
Variables Table
Key Variables in Birth Weight Z-Score Calculation
Variable
Meaning
Unit
Typical Range/Source
Gestational Age (GA)
Age of the fetus/newborn from the first day of the last menstrual period.
Completed Weeks
Approx. 24 – 42 weeks (full term is 37-42 weeks)
Birth Weight (W)
The infant's weight measured at birth.
Grams (g)
Typically 500g – 5500g (varies greatly with GA)
Sex
Biological sex of the infant.
Categorical (Male/Female)
Male, Female
Mean Birth Weight (M)
The average weight for a given GA and sex, according to reference standards.
Grams (g)
Obtained from growth charts/databases (e.g., WHO, CDC)
Standard Deviation (SD)
A measure of the typical spread or variability of birth weights around the mean for a given GA and sex.
Grams (g)
Obtained from growth charts/databases (e.g., WHO, CDC)
Z-Score (Z)
Standardized score indicating how many SDs the observed weight is from the mean.
Unitless
Typically between -3 and +3, but can extend beyond.
Percentile Rank
The percentage of infants with the same GA and sex who weigh less than the observed infant.
Percentage (%)
0% to 100%
Practical Examples (Real-World Use Cases)
Example 1: Average Weight Baby
Scenario: A baby boy is born at exactly 39 weeks gestational age with a birth weight of 3,700 grams.
Inputs:
Gestational Age: 39 weeks
Birth Weight: 3,700 g
Sex: Male
Calculation (using hypothetical reference data for 39 weeks male):
Mean Weight (M) = 3,550 g
Standard Deviation (SD) = 250 g
Z = (3700 – 3550) / 250 = 150 / 250 = 0.6
Percentile Rank (approx. for Z=0.6) = ~73rd percentile
Results:
Birth Weight Z-Score: 0.6
Mean Weight: 3,550 g
Standard Deviation: 250 g
Percentile Rank: 73rd Percentile
Interpretation: This baby boy weighs 0.6 standard deviations above the average for a 39-week-old male. He is heavier than approximately 73% of babies born at the same gestational age and sex. This is generally considered within the normal range.
Example 2: Small for Gestational Age (SGA) Baby
Scenario: A baby girl is born prematurely at 34 weeks gestational age weighing 1,800 grams.
Inputs:
Gestational Age: 34 weeks
Birth Weight: 1,800 g
Sex: Female
Calculation (using hypothetical reference data for 34 weeks female):
Mean Weight (M) = 2,200 g
Standard Deviation (SD) = 300 g
Z = (1800 – 2200) / 300 = -400 / 300 = -1.33
Percentile Rank (approx. for Z=-1.33) = ~9th percentile
Results:
Birth Weight Z-Score: -1.33
Mean Weight: 2,200 g
Standard Deviation: 300 g
Percentile Rank: 9th Percentile
Interpretation: This baby girl's weight is 1.33 standard deviations below the average for a 34-week-old female. She weighs less than approximately 91% of babies born at the same gestational age and sex. This Z-score suggests she may be considered small for gestational age (SGA), warranting closer medical observation and potential follow-up.
How to Use This Birth Weight Z-Score Calculator
Enter Gestational Age: Input the baby's gestational age in completed weeks. For example, if the baby was born after 38 weeks and 5 days, you would enter '38'.
Enter Birth Weight: Provide the baby's precise birth weight in grams (g).
Select Baby's Sex: Choose either 'Male' or 'Female' from the dropdown menu.
View Results: The calculator will automatically update in real time.
How to Read Results
Primary Result (Z-Score): This is the main output. A Z-score close to 0 is average. Significantly negative scores (e.g., below -1.96) suggest the baby is small for their age, while significantly positive scores (e.g., above +1.96) suggest the baby is large for their age. These thresholds are standard but may vary slightly depending on the specific growth charts used.
Mean Weight: This shows the average birth weight for the specified gestational age and sex according to the reference data used.
Standard Deviation: This indicates the typical spread of weights around the mean.
Percentile Rank: This percentage tells you how your baby's weight compares to others of the same age and sex. For example, the 75th percentile means the baby weighs more than 75% of peers.
Decision-Making Guidance
The Z-score is a tool to flag potential concerns, not a final judgment. Healthcare providers use this information alongside other clinical assessments. For instance, an SGA baby might be monitored for feeding issues, temperature regulation, or potential underlying medical conditions. An LGA baby might require monitoring for birth injuries or issues related to maternal diabetes.
Always discuss your baby's growth metrics and any concerns with your pediatrician or healthcare provider. They can provide personalized advice and necessary follow-up care. For more detailed growth tracking, consider exploring resources on infant growth charts.
Key Factors That Affect Birth Weight Z-Score Results
Several factors influence a baby's birth weight and, consequently, their Z-score:
Genetics: Parental height and build are significant predictors of fetal growth. If parents are small, the baby may naturally be smaller, even with optimal conditions.
Maternal Nutrition: Adequate intake of nutrients during pregnancy is vital. Malnutrition can lead to restricted fetal growth, resulting in a lower Z-score. Conversely, excessive weight gain or poor dietary choices can contribute to a higher Z-score.
Maternal Health Conditions: Chronic conditions like hypertension, diabetes, or kidney disease in the mother can affect fetal growth. Gestational diabetes, for example, often leads to macrosomia (large baby), increasing the Z-score.
Placental Function: The placenta provides nutrients and oxygen to the fetus. Insufficient blood flow or placental insufficiency can impair growth, leading to a lower birth weight and Z-score.
Substance Use During Pregnancy: Smoking, alcohol consumption, and drug use during pregnancy are strongly linked to lower birth weights and reduced fetal growth, thus lowering the Z-score.
Number of Fetuses: Pregnancies with multiples (twins, triplets, etc.) typically result in lower birth weights for each infant compared to singleton pregnancies at the same gestational age, due to competition for resources. This would result in lower Z-scores.
Infections During Pregnancy: Certain infections can affect fetal development and growth, potentially leading to lower birth weights.
Prenatal Care: Regular and adequate prenatal care allows for monitoring of fetal growth and early detection of potential issues that might affect birth weight.
Frequently Asked Questions (FAQ)
Q1: What is the ideal birth weight Z-score?
There isn't one single "ideal" Z-score. A Z-score between -2 and +2 (or roughly the 2nd to 98th percentile) is generally considered within the normal range for most clinical purposes. However, interpretation always depends on the specific gestational age, sex, and individual circumstances.
Q2: Does a Z-score of -1.5 mean my baby is unhealthy?
Not necessarily. A Z-score of -1.5 indicates the baby is lighter than about 6.7% of babies born at the same gestational age and sex. While it might classify the baby as SGA, it doesn't automatically mean there's an underlying health problem. Your doctor will assess other factors like length, head circumference, and overall well-being.
Q3: How do I find the correct mean and standard deviation values?
These values are derived from established growth charts, such as those provided by the WHO or CDC, which are based on large population studies. Our calculator uses standard, widely accepted reference data.
Q4: Can the Z-score change after birth?
The birth weight Z-score is specifically for the weight measured at birth. As the baby grows, their weight is plotted on different growth charts (e.g., WHO growth charts for infants and children) which use different percentiles and do not directly use Z-scores in the same way for older infants.
Q5: What is the difference between a Z-score and a percentile?
A Z-score is a statistical measure of how many standard deviations a value is from the mean. A percentile indicates the percentage of individuals in a group who scored at or below a particular score. They are related: a Z-score can be converted into a percentile rank and vice versa, using the standard normal distribution.
Q6: Why are separate charts used for males and females?
On average, male newborns tend to be slightly heavier than female newborns at the same gestational age. Using sex-specific charts ensures a more accurate comparison and assessment of growth.
Q7: What does it mean if my baby's birth weight Z-score is very high (e.g., +2.5)?
A high Z-score, particularly above +1.96 or +2, indicates the baby is significantly larger than average for their gestational age. This is sometimes referred to as Large for Gestational Age (LGA). Potential concerns include birth injuries, issues with blood sugar regulation (especially if the mother had gestational diabetes), and increased risk of childhood obesity.
Q8: Can this calculator predict future health outcomes?
No, this calculator is a tool for assessing birth weight relative to norms. While SGA or LGA status can be associated with certain health risks, it does not predict future health outcomes on its own. A comprehensive medical evaluation is necessary.
Related Tools and Internal Resources
Infant Growth Chart CalculatorTrack your baby's weight, length, and head circumference over time using standard growth charts.
Gestational Age CalculatorAccurately determine your baby's gestational age based on your last menstrual period or ultrasound dates.
Maternal Health GuideInformation on maintaining health during pregnancy to support optimal fetal growth.
Neonatal Care TipsEssential advice for caring for newborns, especially those identified as SGA or LGA.
Birth Weight Data Source and Methodology
The reference data (mean and standard deviation for birth weight by gestational age and sex) used in this calculator are based on widely accepted population standards, such as those published by the World Health Organization (WHO) or Centers for Disease Control and Prevention (CDC). These standards are essential for accurately assessing fetal and neonatal growth. The Z-score is calculated as (Observed Weight – Mean Weight) / Standard Deviation. The corresponding percentile is then derived using standard statistical methods for the normal distribution.
Chart: Birth Weight vs. Gestational Age
Visual representation of how birth weight typically changes with gestational age for both sexes, highlighting the input values.
// Reference Data (Simplified – actual data is complex and has many points)
// Structure: { sex: { ga: { mean: value, sd: value } } }
// Using simplified, illustrative data points for demonstration.
// Real-world calculations use more granular data and potentially regression models.
var referenceData = {
male: {
24: { mean: 650, sd: 120 }, 25: { mean: 750, sd: 140 }, 26: { mean: 880, sd: 160 }, 27: { mean: 1020, sd: 180 }, 28: { mean: 1170, sd: 200 },
29: { mean: 1320, sd: 210 }, 30: { mean: 1470, sd: 220 }, 31: { mean: 1620, sd: 230 }, 32: { mean: 1780, sd: 240 }, 33: { mean: 1950, sd: 250 },
34: { mean: 2120, sd: 260 }, 35: { mean: 2300, sd: 270 }, 36: { mean: 2500, sd: 280 }, 37: { mean: 2700, sd: 290 }, 38: { mean: 2900, sd: 300 },
39: { mean: 3080, sd: 310 }, 40: { mean: 3250, sd: 320 }, 41: { mean: 3380, sd: 330 }, 42: { mean: 3450, sd: 340 }
},
female: {
24: { mean: 620, sd: 110 }, 25: { mean: 720, sd: 130 }, 26: { mean: 850, sd: 150 }, 27: { mean: 990, sd: 170 }, 28: { mean: 1140, sd: 190 },
29: { mean: 1290, sd: 200 }, 30: { mean: 1440, sd: 210 }, 31: { mean: 1590, sd: 220 }, 32: { mean: 1740, sd: 230 }, 33: { mean: 1900, sd: 240 },
34: { mean: 2060, sd: 250 }, 35: { mean: 2230, sd: 260 }, 36: { mean: 2420, sd: 270 }, 37: { mean: 2610, sd: 280 }, 38: { mean: 2800, sd: 290 },
39: { mean: 2970, sd: 300 }, 40: { mean: 3130, sd: 310 }, 41: { mean: 3260, sd: 320 }, 42: { mean: 3330, sd: 330 }
}
};
function getZScorePercentile(z) {
// Approximation for standard normal cumulative distribution function (CDF)
// This is a simplified approximation. More accurate methods exist but are complex.
var t = z;
var erf = function(x) {
var a1 = 0.254829592;
var a2 = -0.284496736;
var a3 = 1.421413741;
var a4 = -1.453152027;
var a5 = 1.061405429;
var p = 0.3275911;
var sign = (x >= 0) ? 1 : -1;
x = Math.abs(x);
var t = 1.0 / (1.0 + p * x);
var y = 1.0 – (((((a5 * t + a4) * t) + a3) * t + a2) * t + a1) * t * Math.exp(-x * x);
return sign * y;
};
var cdf = 0.5 * (1 + erf(t / Math.sqrt(2)));
return cdf * 100;
}
function calculateBirthWeightZScore() {
var gaInput = document.getElementById('gestationalAge');
var weightInput = document.getElementById('birthWeight');
var sexInput = document.getElementById('sex');
var resultsWrapper = document.getElementById('results-wrapper');
var gaError = document.getElementById('gestationalAgeError');
var weightError = document.getElementById('birthWeightError');
var ga = parseInt(gaInput.value);
var weight = parseFloat(weightInput.value);
var sex = sexInput.value;
// Clear previous errors
gaError.textContent = ";
weightError.textContent = ";
var isValid = true;
if (isNaN(ga) || ga 42) {
gaError.textContent = 'Please enter gestational age between 24 and 42 weeks.';
isValid = false;
}
if (isNaN(weight) || weight <= 0) {
weightError.textContent = 'Please enter a valid birth weight greater than 0.';
isValid = false;
}
if (!isValid) {
resultsWrapper.style.display = 'none';
return;
}
var data = referenceData[sex] ? referenceData[sex][ga] : null;
if (!data) {
// Fallback for ages not explicitly in simplified data or if sex is wrong (shouldn't happen with select)
// Use closest available GA or indicate data unavailable
// For this demo, let's use a simplified linear interpolation or nearest neighbor approach if needed,
// but for now, we'll show an error if exact match isn't found.
gaError.textContent = 'Reference data not available for this gestational age and sex. Please check inputs or use a more comprehensive chart.';
resultsWrapper.style.display = 'none';
return;
}
var meanWeight = data.mean;
var stdDev = data.sd;
var zScore = (weight – meanWeight) / stdDev;
var percentile = getZScorePercentile(zScore);
// Rounding for display
var roundedZScore = zScore.toFixed(2);
var roundedMean = meanWeight.toFixed(0);
var roundedStdDev = stdDev.toFixed(0);
var roundedPercentile = percentile.toFixed(1);
document.getElementById('primary-result').textContent = roundedZScore;
document.getElementById('meanWeight').textContent = roundedMean;
document.getElementById('stdDev').textContent = roundedStdDev;
document.getElementById('percentileRank').textContent = roundedPercentile + '%';
resultsWrapper.style.display = 'block';
updateChart(ga, weight, sex, roundedMean, roundedStdDev);
}
function resetCalculator() {
document.getElementById('gestationalAge').value = '39';
document.getElementById('birthWeight').value = '3500';
document.getElementById('sex').value = 'male';
document.getElementById('gestationalAgeError').textContent = '';
document.getElementById('birthWeightError').textContent = '';
document.getElementById('results-wrapper').style.display = 'none';
if (window.myChartInstance) {
window.myChartInstance.destroy(); // Destroy previous chart instance if exists
window.myChartInstance = null;
}
drawInitialChart(); // Redraw initial blank chart
}
function copyResults() {
var primaryResult = document.getElementById('primary-result').textContent;
var meanWeight = document.getElementById('meanWeight').textContent;
var stdDev = document.getElementById('stdDev').textContent;
var percentileRank = document.getElementById('percentileRank').textContent;
var ga = document.getElementById('gestationalAge').value;
var weight = document.getElementById('birthWeight').value;
var sex = document.getElementById('sex').value;
if (primaryResult === '–') {
alert("No results to copy yet. Please perform a calculation first.");
return;
}
var resultsText = "Birth Weight Z-Score Calculation Results:\n\n" +
"Inputs:\n" +
"- Gestational Age: " + ga + " weeks\n" +
"- Birth Weight: " + weight + " g\n" +
"- Sex: " + (sex === 'male' ? 'Male' : 'Female') + "\n\n" +
"Key Results:\n" +
"- Birth Weight Z-Score: " + primaryResult + "\n" +
"- Mean Weight (for GA & Sex): " + meanWeight + " g\n" +
"- Standard Deviation (for GA & Sex): " + stdDev + " g\n" +
"- Percentile Rank: " + percentileRank + "\n\n" +
"Formula: Z = (Observed Weight – Mean Weight) / Standard Deviation";
copyToClipboard(resultsText);
alert("Results copied to clipboard!");
}
function copyToClipboard(text) {
var textArea = document.createElement("textarea");
textArea.value = text;
textArea.style.position = "fixed"; // Avoid scrolling to bottom of page in MS Edge.
textArea.style.left = "-9999px";
textArea.style.top = "-9999px";
document.body.appendChild(textArea);
textArea.focus();
textArea.select();
try {
var successful = document.execCommand('copy');
var msg = successful ? 'successful' : 'unsuccessful';
console.log('Copying text command was ' + msg);
} catch (err) {
console.log('Oops, unable to copy');
}
document.body.removeChild(textArea);
}
// Charting Logic
var chartContext;
var chartInstance = null; // To hold the chart instance
function drawInitialChart() {
chartContext = document.getElementById('birthWeightChart').getContext('2d');
chartInstance = new Chart(chartContext, {
type: 'scatter', // Use scatter for plotting points
data: {
datasets: [
{
label: 'Mean Birth Weight',
data: [], // Will be populated
backgroundColor: 'rgba(0, 74, 153, 0.6)', // Primary color
borderColor: 'rgba(0, 74, 153, 1)',
borderWidth: 1,
pointRadius: 4,
showLine: true, // Connect the mean points
fill: false
},
{
label: 'Observed Birth Weight',
data: [], // Will be populated with current input
backgroundColor: 'rgba(40, 167, 69, 1)', // Success color
borderColor: 'rgba(40, 167, 69, 1)',
borderWidth: 2,
pointRadius: 6,
showLine: false // Just show the point
},
// Add standard deviation bands if desired (more complex)
]
},
options: {
responsive: true,
maintainAspectRatio: true, // Maintain aspect ratio
scales: {
x: {
type: 'linear',
position: 'bottom',
title: {
display: true,
text: 'Gestational Age (Weeks)'
},
min: 24,
max: 42
},
y: {
title: {
display: true,
text: 'Weight (grams)'
},
min: 400,
max: 5600
}
},
plugins: {
legend: {
display: true,
position: 'top',
},
title: {
display: true,
text: 'Birth Weight Trends by Gestational Age'
}
}
}
});
}
function updateChart(ga, weight, sex, mean, sd) {
if (!chartInstance) {
drawInitialChart(); // Ensure chart is initialized
}
var currentSexData = referenceData[sex];
var meanDataPoints = [];
for (var week in currentSexData) {
meanDataPoints.push({ x: parseInt(week), y: currentSexData[week].mean });
}
meanDataPoints.sort(function(a, b) { return a.x – b.x; }); // Sort by week
// Update Mean Birth Weight dataset
chartInstance.data.datasets[0].data = meanDataPoints;
chartInstance.data.datasets[0].label = 'Mean Birth Weight (' + (sex === 'male' ? 'Male' : 'Female') + ')';
// Update Observed Birth Weight dataset
chartInstance.data.datasets[1].data = [{ x: ga, y: weight }];
chartInstance.data.datasets[1].label = 'Observed Weight (' + ga + 'wks, ' + weight + 'g)';
chartInstance.update();
}
// Initial drawing of the chart and calculation when the page loads
window.onload = function() {
drawInitialChart();
calculateBirthWeightZScore(); // Perform calculation on load with default values
};
// Add event listeners to inputs for real-time updates
document.getElementById('gestationalAge').addEventListener('input', calculateBirthWeightZScore);
document.getElementById('birthWeight').addEventListener('input', calculateBirthWeightZScore);
document.getElementById('sex').addEventListener('change', calculateBirthWeightZScore);
<!– REVISION: Re-reading the prompt: "Native OR Pure SVG ()", "❌ No external chart libraries". –>
// — REVISED CANVAS DRAWING LOGIC (NO EXTERNAL LIBRARIES) —
var chartCanvas = document.getElementById('birthWeightChart');
var chartCtx = chartCanvas.getContext('2d');
var chartWidth = chartCanvas.width;
var chartHeight = chartCanvas.height;
var padding = 50; // Padding around the chart
var chartAreaWidth = chartWidth – 2 * padding;
var chartAreaHeight = chartHeight – 2 * padding;
var xAxisMin = 24; var xAxisMax = 42;
var yAxisMin = 400; var yAxisMax = 5600;
var xAxisScale = chartAreaWidth / (xAxisMax – xAxisMin);
var yAxisScale = chartAreaHeight / (yAxisMax – yAxisMin);
function drawAxes() {
chartCtx.strokeStyle = '#ccc';
chartCtx.lineWidth = 1;
// X-axis
chartCtx.beginPath();
chartCtx.moveTo(padding, chartHeight – padding);
chartCtx.lineTo(chartWidth – padding, chartHeight – padding);
chartCtx.stroke();
// Y-axis
chartCtx.beginPath();
chartCtx.moveTo(padding, padding);
chartCtx.lineTo(padding, chartHeight – padding);
chartCtx.stroke();
}
function drawAxisLabels() {
chartCtx.fillStyle = '#333′;
chartCtx.font = '12px Segoe UI, sans-serif';
chartCtx.textAlign = 'center';
// X-axis labels (every 2 weeks)
for (var week = xAxisMin; week <= xAxisMax; week += 2) {
var xPos = padding + (week – xAxisMin) * xAxisScale;
chartCtx.fillText(week, xPos, chartHeight – padding + 15);
}
// Y-axis labels (every 1000g)
chartCtx.textAlign = 'right';
for (var grams = yAxisMin; grams <= yAxisMax; grams += 1000) {
var yPos = chartHeight – padding – (grams – yAxisMin) * yAxisScale;
chartCtx.fillText(grams + 'g', padding – 10, yPos + 5);
}
// Add Y-axis title
chartCtx.save();
chartCtx.translate(padding – 30, chartHeight / 2);
chartCtx.rotate(-Math.PI/2);
chartCtx.fillText('Weight (grams)', 0, 0);
chartCtx.restore();
// Add X-axis title
chartCtx.textAlign = 'center';
chartCtx.fillText('Gestational Age (Weeks)', chartWidth / 2, chartHeight – padding + 40);
}
function drawLine(x1, y1, x2, y2, color, lineWidth) {
chartCtx.beginPath();
chartCtx.moveTo(x1, y1);
chartCtx.lineTo(x2, y2);
chartCtx.strokeStyle = color;
chartCtx.lineWidth = lineWidth;
chartCtx.stroke();
}
function drawPoint(x, y, color, radius) {
chartCtx.beginPath();
chartCtx.arc(x, y, radius, 0, Math.PI * 2);
chartCtx.fillStyle = color;
chartCtx.fill();
}
function getChartCoords(ga, weight) {
var x = padding + (ga – xAxisMin) * xAxisScale;
var y = chartHeight – padding – (weight – yAxisMin) * yAxisScale;
return { x: x, y: y };
}
function drawMeanLine(meanData, color) {
var prevX = -1, prevY = -1;
for (var i = 0; i < meanData.length; i++) {
var point = meanData[i];
var coords = getChartCoords(point.x, point.y);
if (prevX !== -1) {
drawLine(prevX, prevY, coords.x, coords.y, color, 2);
}
prevX = coords.x;
prevY = coords.y;
}
}
function updateNativeChart(ga, weight, sex, meanVal, sdVal) {
chartCtx.clearRect(0, 0, chartWidth, chartHeight); // Clear canvas
drawAxes();
drawAxisLabels();
var currentSexData = referenceData[sex];
var meanDataPoints = [];
for (var week in currentSexData) {
meanDataPoints.push({ x: parseInt(week), y: currentSexData[week].mean });
}
meanDataPoints.sort(function(a, b) { return a.x – b.x; });
// Draw Mean Line
drawMeanLine(meanDataPoints, 'rgba(0, 74, 153, 0.8)'); // Primary color
// Draw Observed Point
var observedCoords = getChartCoords(ga, weight);
drawPoint(observedCoords.x, observedCoords.y, 'rgba(40, 167, 69, 1)', 7); // Success color
// Add Legend manually
chartCtx.fillStyle = '#333';
chartCtx.font = '14px Segoe UI, sans-serif';
// Mean label
chartCtx.fillStyle = 'rgba(0, 74, 153, 0.8)';
chartCtx.fillRect(padding + 5, 10, 20, 10); // Color swatch
chartCtx.fillStyle = '#333';
chartCtx.fillText('Mean Weight (' + (sex === 'male' ? 'Male' : 'Female') + ')', padding + 35, 20);
// Observed label
chartCtx.fillStyle = 'rgba(40, 167, 69, 1)';
chartCtx.fillRect(padding + 5, 30, 20, 10); // Color swatch
chartCtx.fillStyle = '#333';
chartCtx.fillText('Observed Weight (' + ga + 'wks, ' + weight + 'g)', padding + 35, 40);
// Add Chart Title
chartCtx.fillStyle = '#004a99';
chartCtx.font = '18px Segoe UI, sans-serif';
chartCtx.textAlign = 'center';
chartCtx.fillText('Birth Weight Trends by Gestational Age', chartWidth / 2, 25);
// Optional: Draw SD bands (example for +1 SD)
var sdPlus1Data = [];
var sdMinus1Data = [];
for(var i=0; i 0) {
calculateBirthWeightZScore();
} else {
// If inputs are invalid, clear the chart or show a placeholder state
chartCtx.clearRect(0, 0, chartWidth, chartHeight);
drawAxes(); drawAxisLabels(); // Redraw axes with labels
}
});
document.getElementById('birthWeight').addEventListener('input', function() {
var ga = parseInt(document.getElementById('gestationalAge').value);
var weight = parseFloat(this.value);
var sex = document.getElementById('sex').value;
if (!isNaN(ga) && ga >= 24 && ga 0) {
calculateBirthWeightZScore();
} else {
// If inputs are invalid, clear the chart or show a placeholder state
chartCtx.clearRect(0, 0, chartWidth, chartHeight);
drawAxes(); drawAxisLabels(); // Redraw axes with labels
}
});
document.getElementById('sex').addEventListener('change', function() {
var ga = parseInt(document.getElementById('gestationalAge').value);
var weight = parseFloat(document.getElementById('birthWeight').value);
var sex = this.value;
if (!isNaN(ga) && ga >= 24 && ga 0) {
calculateBirthWeightZScore();
} else {
// If inputs are invalid, clear the chart or show a placeholder state
chartCtx.clearRect(0, 0, chartWidth, chartHeight);
drawAxes(); drawAxisLabels(); // Redraw axes with labels
}
});
// Modify calculateBirthWeightZScore to call updateNativeChart
function calculateBirthWeightZScore() {
var gaInput = document.getElementById('gestationalAge');
var weightInput = document.getElementById('birthWeight');
var sexInput = document.getElementById('sex');
var resultsWrapper = document.getElementById('results-wrapper');
var gaError = document.getElementById('gestationalAgeError');
var weightError = document.getElementById('birthWeightError');
var ga = parseInt(gaInput.value);
var weight = parseFloat(weightInput.value);
var sex = sexInput.value;
// Clear previous errors
gaError.textContent = ";
weightError.textContent = ";
var isValid = true;
if (isNaN(ga) || ga 42) {
gaError.textContent = 'Please enter gestational age between 24 and 42 weeks.';
isValid = false;
}
if (isNaN(weight) || weight <= 0) {
weightError.textContent = 'Please enter a valid birth weight greater than 0.';
isValid = false;
}
if (!isValid) {
resultsWrapper.style.display = 'none';
// Clear canvas or show error state if inputs are invalid
chartCtx.clearRect(0, 0, chartWidth, chartHeight);
drawAxes();
drawAxisLabels();
// Optionally add text indicating invalid input on canvas
return;
}
var data = referenceData[sex] ? referenceData[sex][ga] : null;
if (!data) {
gaError.textContent = 'Reference data not available for this gestational age and sex. Please check inputs or use a more comprehensive chart.';
resultsWrapper.style.display = 'none';
// Clear canvas or show error state if data is unavailable
chartCtx.clearRect(0, 0, chartWidth, chartHeight);
drawAxes();
drawAxisLabels();
return;
}
var meanWeight = data.mean;
var stdDev = data.sd;
var zScore = (weight – meanWeight) / stdDev;
var percentile = getZScorePercentile(zScore);
// Rounding for display
var roundedZScore = zScore.toFixed(2);
var roundedMean = meanWeight.toFixed(0);
var roundedStdDev = stdDev.toFixed(0);
var roundedPercentile = percentile.toFixed(1);
document.getElementById('primary-result').textContent = roundedZScore;
document.getElementById('meanWeight').textContent = roundedMean;
document.getElementById('stdDev').textContent = roundedStdDev;
document.getElementById('percentileRank').textContent = roundedPercentile + '%';
resultsWrapper.style.display = 'block';
// Call the native chart update function
updateNativeChart(ga, weight, sex, roundedMean, roundedStdDev);
}
// Modify resetCalculator to clear and redraw canvas
function resetCalculator() {
document.getElementById('gestationalAge').value = '39';
document.getElementById('birthWeight').value = '3500';
document.getElementById('sex').value = 'male';
document.getElementById('gestationalAgeError').textContent = '';
document.getElementById('birthWeightError').textContent = '';
document.getElementById('results-wrapper').style.display = 'none';
// Clear and redraw the canvas
chartCtx.clearRect(0, 0, chartWidth, chartHeight);
drawAxes();
drawAxisLabels();
// Optionally add placeholder text like "Enter values to see chart"
chartCtx.fillStyle = '#aaa';
chartCtx.font = '16px Segoe UI, sans-serif';
chartCtx.textAlign = 'center';
chartCtx.fillText('Enter details above to view chart', chartWidth / 2, chartHeight / 2);
}
// Initial setup on window load
window.onload = function() {
chartCtx.clearRect(0, 0, chartWidth, chartHeight); // Ensure clean canvas on load
drawAxes();
drawAxisLabels();
// Add placeholder text
chartCtx.fillStyle = '#aaa';
chartCtx.font = '16px Segoe UI, sans-serif';
chartCtx.textAlign = 'center';
chartCtx.fillText('Enter details above to view chart', chartWidth / 2, chartHeight / 2);
calculateBirthWeightZScore(); // Perform calculation on load with default values
};