Climatic and ethnic modifiers of fetal biometric parameters: a comparative regional analysis

Climatic fluctuations and the ethnic heterogeneity of the population can shift the standards of intrauterine growth; however, their combined effect has been little studied. Air temperature, day length, and pregnancy season, in combination with the mother's ethnicity, modify fetal biometry and newborn weight. A retrospective cohort analysis included 1,812 singleton pregnancies completed between 2018 and 2024. Fetal biometric parameters (BPD, OFD, HC, AC, FL) were measured according to unified ultrasound screening protocols. Climatic data by trimester (mean temperature, number of sunny days, photoperiod duration) were obtained from Roshydromet. Ethnic groups: Russians, Tatars, Mari, Udmurts. Two-way ANOVA and multilevel linear modeling adjusted for gestational age were used. The third trimester of pregnancy, occurring in the spring–summer period (+4°C above annual average), was associated with a 3.1% increase in abdominal circumference (p < 0.01) and an increase in newborn weight by 112 g. The ethnic factor manifested from week 24: Finno-Ugric groups showed a significant reduction in femur length (–2.4 mm) with a simultaneous increase in AC (+6.5 mm). In the newborn weight model, climatic predictors explained 18% of the variance, ethnicity 7%, and their interaction 4%. Furthermore, accounting for these factors improved the accuracy of the Estimated Fetal Weight prediction model by 9% compared to the standard Hadlock formula. Climatic conditions and ethnocultural background form independent and interdependent trajectories of intrauterine growth, cumulatively accounting for about one-third of the variance in birth weight. These data substantiate the need for seasonal and ethnic adjustments when assessing fetal biometry.