First Trimester Anatomy Ultrasound for Obese Patients: a Randomized Controlled Trial.

OBJECTIVE: Second-trimester ultrasound is the standard technique for fetal anatomy evaluation in the United States despite international guidelines and literature that suggest that first-trimester timing may be superior in obese patients. First-trimester imaging performs well in cohorts of obese participants. Our aim was to compare the completion rate of first-trimester fetal anatomy to second-trimester fetal anatomy ultrasound in pregnant people with body mass index (BMI) ≥ 35 kg/m2 .

METHODS: This randomized controlled trial enrolled participants with BMI ≥ 35 kg/m2 with singleton gestation, presenting before 14 0/7 weeks of gestation. Participants were randomized to receive ultrasound assessment of anatomy at either 12 0/7 to 13 6/7 weeks, or at 18 0/7 to 22 6/7 weeks. The primary outcome was completion rate (percent of scans that optimally imaged all required fetal structures). Secondary outcomes included the necessity of a transvaginal approach, completion rates of each individual view, number of anomalies identified and missed in each group, scan duration, and patient perspectives. A one-year pilot sample was analyzed using Bayesian methods for the primary outcome with a neutral prior, and frequentist analyses for the remaining outcomes.

RESULTS: A total of 128 participants enrolled, 1 withdrew consent; 62 subjects received a first-trimester ultrasound and 62 received a second-trimester ultrasound; 2 did not attend research visits, and 1 sought termination of pregnancy. In the first-trimester group, 66% (41/62) of ultrasounds were completed compared to 53% (33/62) in the second-trimester ultrasound group (Bayesian relative risk (RR) 1.20, 95% credible interval 0.91-1.73). Compared to a second-trimester scan plus one follow-up ultrasound, a first-trimester ultrasound plus a second-trimester ultrasound was equally successful for completing anatomy (76%). First-trimester anatomy ultrasounds required a transvaginal approach in 63% (39/62) of cases and had a longer duration compared to a second-trimester ultrasounds. No anomalies were missed in either group. First-trimester ultrasound participants who responded to a survey described that they were very satisfied with the technique.

CONCLUSIONS: Inpregnant subjects with BMI>35, a single first-trimester anatomy ultrasound is more likely to obtain all recommended anatomic views compared to a single second-trimester ultrasound. An evaluation of anatomy at 12 0/7 to 13 6/7 weeks, plus an evaluation at 18 0/7 to 22 6/7 resulted in complete evaluation 4 weeks earlier than two second trimester scans. Assessment of ultrasound duration in a clinical setting is needed to ensure feasibility outside of a research setting.