Greater maternal weight gain during pregnancy predicts a large but lean fetal phenotype: a prospective cohort study.

The objective of this study is to describe the fetal phenotype in utero and its associations with maternal pre-pregnancy weight and gestational weight gain. This prospective longitudinal cohort included 179 Australian women with singleton pregnancies. Serial ultrasound measurements were performed at 19, 25, 30 and 36 (±1) weeks gestation and maternal anthropometry were collected concurrently. The ultrasound scans included the standard fetal biometry of head circumference, biparietal diameter, abdominal circumference, and femur length, and body composition at the abdomen and mid-thigh, including fat and lean mass cross-sectional areas. Maternal gestational weight gain was compared to current clinical guidelines. The participants had an average of 3.7 ± 0.8 scans and birth data were available for 165 neonates. Fifty four per cent of the cohort gained weight in excess of current recommendations, according to pre-pregnancy body mass index (BMI). Maternal gestational weight positively predicted fetal abdominal circumference (P 0.029) and lean abdominal mass area (P 0.046) in linear mixed model regression analysis, adjusted for known and potential confounders. At any pre-pregnancy BMI gaining weight above the current recommendations resulted in a larger fetus according to standard biometry, because of significantly larger lean muscle mass at the abdomen (P 0.024) and not due to an increase in fat mass (P 0.463). We have demonstrated the importance of maternal weight gain, independent of pre-pregnancy BMI, to support the growth of a large but lean fetus. Prenatal counselling should focus on achieving a healthy BMI prior to conception so that gestational weight gain restrictions can be minimised.