Maternal obesity is a potential source of error in mid-trimester ultrasound estimation of gestational age.

OBJECTIVES: To investigate the possible impact of maternal obesity on ultrasonographic dating of pregnancy.

METHODS: The Swedish Medical Birth Registry was used to identify 842 083 singleton pregnancies for which the estimated date of delivery (EDD) according to last menstrual period (EDD-LMP) and according to ultrasound examination (EDD-US) and maternal body mass index (BMI) in early pregnancy were known. Dates were adjusted following ultrasound examination, negative adjustment representing pregnancies that were shorter according to ultrasound examination than they were according to LMP. Odds ratios (OR) were adjusted for year of birth, maternal age, parity and smoking.

RESULTS: The EDD was postponed at least 7 days (discrepancy between EDD-LMP and EDD-US of < or = -7 days) in 25.2% of all pregnancies. There was a statistically significant association between maternal BMI and discrepancy between EDD-LMP and EDD-US. Among pregnant women with a BMI > or = 30.0 kg/m(2), the risk of postponed EDD was significantly greater compared with women with a BMI of 20.0-24.9 kg/m(2). The EDD was more often postponed > or = 14 days (OR, 1.65; 95% CI, 1.60-1.70) or 7-13 days (OR, 1.45; 95% CI, 1.42-1.48) among women with a BMI > or = 30 kg/m(2) than among women with normal weight. Similar risk was observed among women with a BMI of 25.0-29.9 kg/m(2), but was less pronounced.

CONCLUSIONS: High maternal BMI increases the risk of postponing the EDD at mid-trimester ultrasound examination.