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Background The increasing birthweight trend stopped and even reversed in several high income countries in the last 20 years, however the reason for these changes is not well characterized. We aimed to analyze the birthweight trends of term deliveries and their potential maternal and fetal drivers in Hungary. Methods We analyzed data from the Hungarian Tauffer registry, a compulsory anonymized data collection of each delivery. We included all singleton term deliveries between in 1999-2018 (n=1 591 932). We modeled birthweight trends separately in 1999-2008 and 2008-2018 in hierarchical multiple linear regression models adjusted for calendar year, newborn sex, maternal age, gestational age at delivery, and other important determinants. Results Median birthweights increased from 3250/3400g (girl/boy) to 3300/3440g from 1999 to 2008 and decreased to 3260/3400g in 2018. When we adjusted for gestational age at delivery the increase in the first period became more pronounced (5.4 g/year). During the second period, similar adjustment substantially decreased the rate of decline from 2.5 to 1.4g/year. Further adjustment for maternal age halved the rate of increase to 2.4g/year in the first period. During the second period, adjustment for maternal age had little effect on the estimate. Conclusions Our findings of an increasing birthweight trend (mostly related to the aging of the mothers) in 1999-2008 may forecast an increased risk of cardiometabolic diseases in offsprings born in this period. In contrast, the decreasing birthweight trends after 2008 may reflect some beneficial effects on perinatal morbidity. However, the long-term effect cannot be predicted, as the trend is mostly explained by the shorter pregnancies.

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