Expected Baby Birth Weight Estimator

The average birth weight for a full-term singleton baby (39–40 weeks) is approximately 3.3–3.5 kg (7 lbs 4 oz – 7 lbs 12 oz). Boys average slightly heavier than girls. The normal range is typically 2.5–4.5 kg.

Low birth weight (LBW) is defined as less than 2,500 g (5 lbs 8 oz) at birth, regardless of gestational age. Very low birth weight (VLBW) is under 1,500 g, and extremely low birth weight (ELBW) is under 1,000 g.

A baby is considered large for gestational age (LGA) if their birth weight falls above the 90th percentile for their gestational age and sex. LGA births are associated with gestational diabetes, maternal obesity, and genetic factors.

Yes, on average. Twin birth weights are typically 15–20% lower than singleton birth weights at the same gestational age because twins share nutrients and placental space. The average twin birth weight is around 2.5 kg each when born near term.

Third-trimester ultrasound measurements (biparietal diameter, femur length, abdominal circumference) can estimate fetal weight but have a margin of error of ±15–20%. The estimate becomes more accurate closer to delivery but remains an approximation.

Most fetal weight gain occurs in the final trimester, particularly after 32 weeks. Before 32 weeks, babies have had less time to accumulate fat stores and muscle mass, resulting in significantly lower birth weights. Preterm guidelines for growth and nutrition differ from those for term babies.