Birth Weight Prediction Calculator

Estimate fetal weight based on gestational age and maternal metrics

Upcoming Weight Milestones

Gestational Age	Est. Weight (g)	Est. Weight (lbs/oz)	Growth Status

Based on current trajectory and maternal factors.

What is a Birth Weight Prediction Calculator?

A birth weight prediction calculator is a digital tool designed for expectant parents and healthcare providers to estimate the current weight of a fetus and project its weight at delivery (usually 40 weeks). Monitoring fetal growth is a critical component of prenatal care, as deviations from normal growth patterns can indicate conditions such as Intrauterine Growth Restriction (IUGR) or Macrosomia (large for gestational age).

Unlike clinical tools that rely on real-time ultrasound measurements like biparietal diameter (BPD) or abdominal circumference (AC), this web-based birth weight prediction calculator uses statistical averages based on Last Menstrual Period (LMP) dating, adjusted for key biometrics like maternal height, pre-pregnancy weight, and fetal biological sex.

While not a replacement for medical imaging, this tool provides a helpful baseline to understand where your baby sits on the growth curve relative to the general population.

Birth Weight Prediction Calculator Formula and Logic

The estimation of fetal weight involves complex regression formulas derived from population studies. Our calculator utilizes a simplified variation of the Hadlock and WHO fetal growth standards, adjusted for maternal characteristics.

Key Variables Used

Variable	Meaning	Unit	Typical Impact
Gestational Age (GA)	Time elapsed since the first day of the last menstrual period.	Weeks + Days	Primary driver of weight. Growth is exponential until ~36 weeks.
Fetal Sex	Biological sex of the fetus.	Male/Female	Males are typically ~150g heavier at term than females.
Maternal BMI	Mother's Body Mass Index derived from height and weight.	kg/m²	Higher maternal BMI correlates with higher birth weights.
Parity	Number of previous births.	Count	Second and subsequent babies are often heavier than firstborns.

The mathematical model approximates the 50th percentile weight ($W_{50}$) for a given week ($t$) using a polynomial function, then applies a multiplier ($M$) for maternal factors:

Estimated Weight = BaseWeight(t) × SexFactor × MaternalFactor

Practical Examples of Weight Prediction

Example 1: The Average Scenario

Scenario: Sarah is 30 weeks pregnant with a female fetus. She had a pre-pregnancy weight of 65kg and is 165cm tall (BMI ~23.9, normal range).

• Input: 30 Weeks, Female, 65kg, 165cm.
• Base Calculation: Average fetus at 30 weeks is ~1300g.
• Adjustments: Female sex adjustment (-3%) reduces this slightly. Maternal BMI is average, so no major skew.
• Output: Approximately 1,280g (2 lbs 13 oz).
• Interpretation: The baby is tracking normally.

Example 2: Factors for Macrosomia

Scenario: Elena is 34 weeks pregnant with a male fetus. Her pre-pregnancy BMI was 30 (obese class I).

• Input: 34 Weeks, Male, High BMI.
• Base Calculation: Average fetus at 34 weeks is ~2100g.
• Adjustments: Male sex adjustment (+3%) and BMI adjustment (+4%) increase the estimate.
• Output: Approximately 2,250g (4 lbs 15 oz).
• Interpretation: The fetus is trending above the 50th percentile, which is common with higher maternal BMI.

How to Use This Birth Weight Prediction Calculator

1. Enter Gestational Age: Select the number of weeks and days since your last period. This is the most important field.
2. Select Fetal Sex: If known, choose Male or Female. If unknown, leave as "Unknown" for a neutral average.
3. Input Maternal Metrics: Enter pre-pregnancy weight (in kg) and height (in cm). These help calculate the Maternal BMI factor.
4. Review Results: Look at the "Estimated Fetal Weight" in the blue box.
5. Check the Chart: The graph displays your baby's estimated trajectory against the standard growth curve.
6. Use the Table: Plan ahead by looking at the "Upcoming Weight Milestones" table to see how big the baby might be in the coming weeks.

Key Factors That Affect Birth Weight Prediction Results

While a birth weight prediction calculator provides a mathematical estimate, biological reality is influenced by at least six major factors:

• Genetics: The height and weight of both parents play a significant role. Tall parents tend to have longer, heavier babies.
• Placental Health: The placenta regulates nutrient flow. Insufficiency can lead to lower birth weights regardless of genetic potential.
• Maternal Health Conditions: Gestational diabetes often leads to larger babies (macrosomia), while hypertension or pre-eclampsia can restrict growth.
• Multiples: Twins and triplets typically have lower individual birth weights and often slow in growth after week 30 due to space constraints.
• Nutrition: Adequate maternal caloric and protein intake is essential for fetal muscle and fat deposition.
• Smoking and Substances: Smoking constricts blood vessels, significantly reducing oxygen flow to the fetus and often resulting in lower birth weights.

Frequently Asked Questions (FAQ)

How accurate is this birth weight prediction calculator?

This calculator uses population averages. While it accounts for maternal BMI and sex, it cannot "see" the baby. Clinical ultrasound weight estimates typically have a margin of error of +/- 15%, and online calculators are estimates based on trends, not diagnosis.

What is considered a low birth weight?

A birth weight of less than 2,500 grams (5 lbs 8 oz) is generally considered Low Birth Weight (LBW). This can be caused by prematurity or growth restriction.

What is considered a high birth weight (Macrosomia)?

A baby weighing more than 4,000 grams (8 lbs 13 oz) is typically considered macrosomic. This is more common in post-term pregnancies or mothers with gestational diabetes.

Does the calculator work for twins?

No. This birth weight prediction calculator assumes a singleton pregnancy. Twins have different growth curves and typically weigh less individually.

Why does the calculator ask for my height and weight?

Maternal body size is a known predictor of fetal size. Higher maternal BMI is statistically associated with heavier babies, while lower BMI is associated with lighter babies.

When does the baby gain the most weight?

Fetal weight gain accelerates significantly in the third trimester (after 28 weeks), with the baby often gaining roughly 200-250 grams per week by week 35.

Can I change my baby's birth weight?

While you cannot control genetics, maintaining a healthy diet, managing blood sugar levels, and avoiding smoking are the best ways to support an optimal birth weight.

How do I convert grams to pounds and ounces?

1 pound = 453.59 grams. 1 ounce = 28.35 grams. Our calculator performs this conversion automatically in the results section.