Estimate your baby likely birth weight range based on parental birth weights, gestational age, birth order, baby sex, and maternal height using hereditary weight models.
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Generated at traitgen.com. Free genetics education. Not medical advice.
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⚠️ Educational only. Probability estimates based on genetic models, not medical advice.
The average birth weight globally has been increasing over the past century, largely due to improved maternal nutrition and reduced infectious disease during pregnancy. However, in some high-income countries, increasing rates of gestational diabetes are also contributing to higher average birth weights, a trend with mixed implications for delivery outcomes.
Birth weight is influenced by both genetic and environmental factors. Studies estimate that approximately 40 to 50 percent of birth weight variation is heritable. Key determinants include parental birth weights, gestational age, maternal size and nutrition, pregnancy complications, and birth order.
Parental birth weights, particularly the mother's, are the strongest genetic predictors of a baby's birth weight. Genome-wide association studies have identified over 60 genetic variants associated with birth weight, including variants in the ADCY5 gene (insulin signalling), CCNL1, and multiple other metabolic pathway genes. Fetal growth is also influenced by imprinted genes where the paternal copy is expressed differently from the maternal copy.
The mother's own birth weight, current height, and pre-pregnancy weight are the strongest predictors of her baby's birth weight, beyond gestational age. This is partly genetic (mothers who were large babies tend to have large babies) and partly physiological (maternal uterine size and nutrient supply capacity). Maternal gestational diabetes can significantly increase birth weight regardless of genetic predisposition.
First-born babies are on average 100 to 150 grams lighter than subsequent babies from the same mother. This is because the uterus and placental blood supply are not yet fully optimised with a first pregnancy. From the second pregnancy onward, placental efficiency typically improves, supporting slightly greater fetal growth rates. This effect is independent of genetics and applies across all birth weight ranges.
Gestational age at birth is the single most powerful determinant of absolute birth weight. A baby born at 36 weeks will weigh significantly less than a genetically identical baby born at 40 weeks simply due to the shorter growth period. For this reason, birth weight comparisons are most meaningful when referenced to gestational age norms, which is why all modern growth charts use gestational-age-adjusted centiles.
The WHO defines normal birth weight as 2,500 grams (2.5 kg) or above for a full-term baby. Low birth weight is defined as under 2,500 g, very low birth weight as under 1,500 g, and extremely low birth weight as under 1,000 g. The average birth weight for a full-term baby is approximately 3,400 g (3.4 kg) for boys and 3,200 g (3.2 kg) for girls globally, with some variation by population.
There is a modest correlation between birth weight and adult height and weight, but it is not strong. Birth weight reflects fetal growth rate during pregnancy, which is heavily influenced by maternal environment. Many large babies grow into average adults, and many small babies catch up to their genetic height potential by age 2. Parental heights are a far better predictor of adult size than birth weight.
Babies weighing over 4 kg (macrosomia) can present delivery challenges and carry slightly increased risk of shoulder dystocia during vaginal delivery. Large birth weight is often associated with maternal gestational diabetes, which is why glucose screening is standard in pregnancy. If a large baby is expected, the obstetric team will plan delivery accordingly. For the baby, macrosomia itself is not associated with long-term health problems once the delivery is safely managed.
Ultrasound-based birth weight estimates in the third trimester have a margin of error of approximately plus or minus 10 to 15 percent, meaning a 3.5 kg estimate could mean a baby between 3.0 and 4.0 kg. This calculator uses hereditary and pregnancy factors for a broader estimate range rather than ultrasound measurements, so it is a genetic probability tool rather than a clinical prediction. Actual birth weight depends on many factors that cannot be captured without direct medical assessment.