OBJECTIVE: The objective of this study is to investigate the effect of maternal obesity on fetal size in first- and second-trimester pregnancies and to determine duration of pregnancy as estimated by a variety of methods.

METHODS: Between 2006 and 2011, a cohort study included (n = 9055) singleton pregnancies that resulted in live birth at Holbaek Hospital in Denmark. This study recorded first- and second-trimester fetal measurements and maternal anthropometry. Characteristics considered included mother's age, parity, height, body mass index (BMI), smoking habits, and sex of child. The correlation between BMI and duration of pregnancy was analyzed by time-to-event analysis and accounted for medical intervention by censoring while correlation of BMI on fetal size was evaluated by multiple regression analysis.

RESULTS: Adjusting for maternal and fetal characteristics, BMI was associated with prolonged pregnancy duration (0.20-0.22 d per kg/m(2) (standard error (SE) 0.02)) when using ultrasound and 0.26 d per kg/m(2) (SE: 0.03) when using last menstrual period. With increasing BMI, fetal biometries in first and second trimester were significantly smaller than expected (0.08 mm per kg/m(2) when measured by crown rump length (SE 0.02)).

CONCLUSIONS: Maternal BMI is correlated to smaller fetal size in early pregnancy and prolongs duration of pregnancy.