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Expected Newborn Birth Weight Calculator

Estimate your baby's birth weight based on gestational week, sex, maternal BMI, and risk factors. Based on Intergrowth-21st data.

Sources: INTERGROWTH-21st — Newborn Size Standards · ACOG Practice Bulletins — Fetal Growth Restriction · Hadlock FP et al. — Fetal Weight Estimation

How to Estimate Newborn Birth Weight

Knowing the expected birth weight is important for preparing for delivery and identifying situations that require medical attention. This estimate is based on epidemiological data and maternal/fetal risk factors.

Factors That Influence Birth Weight

Birth weight depends on numerous factors:

  • Gestational week: the most important factor. Each additional week at term adds about 150-200 grams
  • Fetal sex: males weigh on average 75-150 grams more than females at the same gestational week
  • Maternal constitution: pre-pregnancy BMI influences fetal weight. Overweight or obese mothers tend to have larger babies
  • Parity: firstborns weigh on average 100-150 grams less than subsequent siblings
  • Smoking: reduces birth weight by 150-250 grams due to placental vasoconstriction
  • Gestational diabetes: increases fetal weight through fetal hyperinsulinemia

Intergrowth-21st Data

This calculator uses reference data from the INTERGROWTH-21st study, the largest international project on fetal and neonatal growth standards, involving over 60,000 pregnancies across 8 countries.

SGA, AGA, and LGA Classification

At birth, newborns are classified by weight relative to gestational age:

  • SGA (Small for Gestational Age): weight below the 10th percentile
  • AGA (Appropriate for Gestational Age): weight between 10th-90th percentile
  • LGA (Large for Gestational Age): weight above the 90th percentile

Frequently Asked Questions

How reliable is the birth weight estimate?▼
This estimate is based on statistical averages and known risk factors. Actual birth weight can vary significantly. Third-trimester ultrasound with Hadlock formulas provides a more precise estimate (average error +/-10-15%). This calculator offers an orientation based on epidemiological factors.
What do SGA, AGA, and LGA mean?▼
SGA (Small for Gestational Age) means weight below the 10th percentile. AGA (Appropriate) is between 10th-90th percentile. LGA (Large for Gestational Age) is above the 90th percentile. SGA and LGA babies require additional monitoring.
Does smoking during pregnancy affect birth weight?▼
Smoking reduces birth weight by an average of 150-250 grams. This effect is dose-dependent. Smoking reduces placental blood flow, limiting oxygen and nutrient supply to the fetus. Quitting within the first trimester significantly reduces this risk.