The impact of obesity on first-trimester pregnancy dating using ultrasound: a retrospective cohort study.

Objectives: Obese women are known to have alterations in their menstrual cycles leading to ovulatory dysfunction. The purpose of this study is to evaluate the relationship between body mass index (BMI) and early pregnancy ultrasound dating and determine the frequency of redating.

Methods: We performed a retrospective cohort study of women who underwent a first trimester dating ultrasound at The University of Oklahoma Health Sciences Center from January 2019 to June 2020. The estimated date of delivery (EDD) calculated from the ultrasound was compared to the EDD calculated from the LMP. A discrepancy of greater than 5 days prior to 9 weeks or greater than 7 days between 9 and 13 weeks prompted redating of the pregnancy.

Results: 2639 women met inclusion criteria, with BMI ≥ 30 kg/m² present in 39% of participants. Obese women had higher rates of pregnancy redating compared to non-obese women-adjusted risk ratio [aRR] 1.23 (95% confidence interval [CI] 1.07-1.41, p = 0.003). Class III obese women were more likely than any other BMI class to have their pregnancies redated by ultrasound-aRR 1.46 (95% CI 1.18-1.80, p < 0.001). Analyses adjusted for age, race, and parity showed that for each unit increase in BMI over 30 kg/m², women had a 2% increased risk of being redated (p < 0.001). Mean EDD by ultrasound minus mean EDD by LMP was -0.98, -2.70, -2.43, and -3.35 (p < 0.001) for non-obese, class I, class II, and class III obese women, respectively. 22% of non-obese women were redated compared to 27.3% of obese women (p = 0.002) and 31.2% of class III obese women (p = 0.001).

Conclusions: Obese women have higher rates of pregnancy redating compared to normal weight women, with class III obese women having the highest rates of all groups studied. Accurate pregnancy dating is important to decrease unnecessary interventions due to false diagnoses of fetal growth restriction or macrosomia.