No influence of body mass index on first trimester fetal growth

Our objective was to determine what effect maternal BMI has on fetal growth rate in the early first trimester.

This was a prospective observational study of singleton pregnancies with certain dates, initially presenting for a transvaginal scan (TVS) before 12 weeks of gestation. Maternal characteristics (BMI, ethnicity, maternal age, obstetric history, abdominal pain and vaginal bleeding) were recorded. Fetal crown-rump length (CRL) was measured at the initial scan, and at subsequent ultrasound assessments. In order to assess the fetal growth rates, women with at least two CRL measurements were included in the analysis. A mixed-linear effects model analysis was performed to determine whether BMI influences the rate of change in CRL.

A total of 264 pregnancies were analysed. The median BMI was 23.55 (range 16-45), median age was 32 (17-44) and the proportion of white, black and Asian women was 61.0, 15.5 and 5.3%, respectively. Mean gestational age (GA) at first TVS was 56 (range 33-84) days. Studying CRL as a function of GA with a mixed-linear effects model showed that this relationship was neither significantly influenced by BMI when modelling BMI as a continuous variable (P = 0.7529), nor when modelling it as a categorical variable using the WHO criteria (P = 0.8904).

Dating by CRL influences subsequent growth assessment and previous studies have suggested that first trimester fetal growth rates may be influenced by ethnicity and age. Our data however suggest that maternal BMI does not significantly influence early fetal growth.