Pregnancy Due Date Calculator Pregnancy Week by Week

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Understanding Your Pregnancy Due Date Calculation

Calculating your estimated due date (EDD) is a crucial part of tracking your pregnancy. While it’s an estimate and only a small percentage of babies are born exactly on their due date, it provides a valuable benchmark for monitoring your baby’s growth and development throughout the trimesters.

How the Due Date is Calculated (Naegele’s Rule and Variations)

The most common method for estimating a due date is based on the first day of your Last Menstrual Period (LMP). This is known as Naegele’s Rule:

• Add 7 days to the first day of your LMP.
• Subtract 3 months from the resulting month.
• Add 1 year to the resulting year.

This rule assumes a standard 28-day cycle with ovulation occurring on day 14. However, not all women have 28-day cycles, and ovulation can occur at different times.

The Role of Cycle Length and Luteal Phase

Our calculator refines this by considering your individual cycle and luteal phase lengths:

• Average Cycle Length: This is the number of days from the start of one period to the start of the next.
• Luteal Phase Length: This is the time from ovulation to the start of your next period. It’s generally more consistent for most women than the follicular phase (the time from the start of your period to ovulation).

The calculation works by:

1. Determining the estimated date of ovulation: LMP Start Date + (Average Cycle Length - Luteal Phase Length) days
2. Adding a standard 266 days (38 weeks) to the estimated ovulation date to arrive at the estimated due date.

This method provides a more personalized estimate than the basic Naegele’s Rule, especially for those with irregular or longer/shorter cycles.

Gestational Age

The gestational age is the length of time from the first day of your LMP to the current date. A full-term pregnancy is typically considered 40 weeks (280 days) from your LMP, or 38 weeks (266 days) from conception (ovulation).

Important Considerations

• This is an Estimate: Due dates are approximate. Only about 4-5% of babies are born on their due date.
• Early Ultrasound: The most accurate dating of pregnancy is often done via an early ultrasound, typically in the first trimester.
• Consult Your Doctor: Always discuss your due date and pregnancy progress with your healthcare provider. This calculator is for informational purposes only and does not replace professional medical advice.
• IVF Pregnancies: If you conceived via IVF, your due date calculation might be based on the date of egg retrieval or embryo transfer. Consult your fertility specialist for precise dating.

When to Use This Calculator

• To get an initial estimate of your due date when you first discover you’re pregnant.
• To track your progress week-by-week throughout your pregnancy.
• To help plan for your baby’s arrival.

function calculateDueDate() {
var lmpDateInput = document.getElementById(“lmpDate”).value;
var cycleLengthInput = document.getElementById(“cycleLength”).value;
var lutealPhaseInput = document.getElementById(“lutealPhase”).value;

var resultElement = document.getElementById(“dueDateResult”);
var gestationalAgeElement = document.getElementById(“gestationalAgeResult”);

if (!lmpDateInput || !cycleLengthInput || !lutealPhaseInput) {
resultElement.innerText = “Please fill in all fields.”;
gestationalAgeElement.innerText = “”;
return;
}

var cycleLength = parseInt(cycleLengthInput);
var lutealPhase = parseInt(lutealPhaseInput);

if (isNaN(cycleLength) || cycleLength 40) {
resultElement.innerText = “Invalid Cycle Length.”;
gestationalAgeElement.innerText = “”;
return;
}
if (isNaN(lutealPhase) || lutealPhase 18) {
resultElement.innerText = “Invalid Luteal Phase.”;
gestationalAgeElement.innerText = “”;
return;
}

var lmpDate = new Date(lmpDateInput);
if (isNaN(lmpDate.getTime())) {
resultElement.innerText = “Invalid LMP Date.”;
gestationalAgeElement.innerText = “”;
return;
}

// Calculate estimated ovulation date
// Days from LMP start to ovulation = Cycle Length – Luteal Phase
var daysToOvulation = cycleLength – lutealPhase;
var ovulationDate = new Date(lmpDate);
ovulationDate.setDate(lmpDate.getDate() + daysToOvulation);

// Calculate estimated due date (266 days from ovulation)
var dueDate = new Date(ovulationDate);
dueDate.setDate(ovulationDate.getDate() + 266);

// Format the due date
var options = { year: ‘numeric’, month: ‘long’, day: ‘numeric’ };
var formattedDueDate = dueDate.toLocaleDateString(‘en-US’, options);

// Calculate gestational age from LMP
var today = new Date();
var diffInTime = today.getTime() – lmpDate.getTime();
var diffInWeeks = Math.floor(diffInTime / (1000 * 60 * 60 * 24 * 7));
var diffInDays = Math.floor((diffInTime / (1000 * 60 * 60 * 24)) % 7);

// Calculate weeks pregnant until due date
var daysUntilDue = Math.floor((dueDate.getTime() – today.getTime()) / (1000 * 60 * 60 * 24));
var weeksUntilDue = Math.floor(daysUntilDue / 7);
var remainingDaysUntilDue = daysUntilDue % 7;

resultElement.innerText = formattedDueDate;
gestationalAgeElement.innerText = “You are approximately ” + diffInWeeks + ” weeks and ” + diffInDays + ” days pregnant. Your due date is in approximately ” + weeksUntilDue + ” weeks and ” + remainingDaysUntilDue + ” days.”;
}