Fetal Weight Percentile Calculator

Fetal weight percentiles are one of the most important metrics in obstetric care. Rather than reporting an absolute weight in grams or ounces, a percentile places your baby's estimated fetal weight (EFW) in context among all fetuses measured at the same gestational age. If your baby is at the 50th percentile, half of all fetuses at that week of pregnancy weigh more and half weigh less. A percentile near the median signals that growth is proceeding as expected for the gestational age, while values at the extremes may prompt closer monitoring. This calculator uses the widely cited Hadlock 1991 growth model. The mathematical foundation is a log-normal distribution: at each gestational age, the natural logarithm of fetal weight is approximately normally distributed with a mean defined by a second-degree polynomial (ln EFW ≈ 0.578 + 0.332 × GA − 0.00354 × GA²) and a standard deviation of about 15% of the mean. A z-score is computed by subtracting the population mean from the observed log weight and dividing by the standard deviation. The standard normal cumulative distribution function then converts that z-score into a percentile rank. Gestational age accuracy is critical. A difference of even one or two weeks can shift the percentile substantially because fetal weight roughly doubles every few weeks during the second and early third trimesters. The most reliable gestational age estimate comes from a first-trimester crown-rump length (CRL) measurement performed before 14 weeks. Later ultrasound dating is less precise, particularly beyond 20 weeks, so clinicians typically lock the gestational age to the earliest reliable estimate. Clinically, three zones guide interpretation. Fetuses below the 10th percentile are classified as small for gestational age (SGA). This group has a higher risk of adverse perinatal outcomes, including preterm birth, neonatal complications, and long-term developmental issues, so they receive more frequent growth scans and fetal Doppler assessments. Fetuses above the 90th percentile are large for gestational age (LGA). LGA is associated with maternal diabetes (both gestational and pre-existing), post-dates pregnancy, and constitutional large size. The majority of fetuses — those between the 10th and 90th percentiles — are classified as appropriate for gestational age (AGA) and generally continue routine monitoring. Twin pregnancies tend to show divergent growth trajectories, particularly after 28 weeks, when placental sharing and uterine space constraints can limit individual fetal growth. This calculator applies a modest adjustment to the expected mean for twin pregnancies after 28 weeks to reflect these physiological differences, though dedicated twin growth charts (such as those from Gratacos or the INTERGROWTH-21st Twin Study) provide greater precision for high-risk multiple gestations. Remember that a single out-of-range percentile does not constitute a diagnosis. Serial measurements plotted over time — typically every three to four weeks — provide far more useful information than a single data point. A fetus consistently tracking at the 8th percentile may simply have small but constitutionally appropriate parents, whereas a fetus that drops from the 45th to the 9th percentile across two measurements represents a significant deceleration in growth that warrants prompt investigation. Always interpret these results in collaboration with a qualified obstetrician or maternal-fetal medicine specialist who has access to the full clinical picture.