Timing the Second Trimester Fetal Ultrasound in the Obese Gravida [37P]

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To identify the ideal time to perform the second trimester ultrasound for fetal assessment in the obese patient population we examined the relationship between scan completion rates and advancing gestational age.


This is a retrospective study at a single tertiary care center. Inclusion criteria were pregnant women with BMI ≥30 who had second trimester fetal ultrasound and delivery at our institution. Exclusion criteria were pregnancies without documented BMI, ultrasound not performed between 18 0/6 and 21 6/7 weeks, multiple gestations, or ultrasound performed for indication other than fetal anatomic assessment. Ultrasound reports were incomplete if they indicated suboptimal or non-visualization of any of 30 fetal anatomic structures. Chi square analysis was used to compare rates of completed surveys.


2,138 records met study criteria. Using 18 weeks as a reference group, the odds of having a complete survey at each gestational age were calculated. At 19 weeks the OR was 1.35 [1.1-1.7, p = 0.001] and at 20 weeks OR 1.37 [1.05-1.8, p = 0.01]. At 21 weeks the difference was no longer statistically significant, OR 1.3 [0.95-1.9]. We found no difference in rates of completed surveys when comparing 19 to 20 weeks, OR 1.015 [0.76-1.34, p = 0.46].


In obese gravidas, the odds of having a complete scan were greatest at 19 and 20 weeks gestational age. Therefore in this population it appears useful to wait until 19 weeks to perform the second trimester fetal ultrasound, but delaying the scan did not further increase the chance of a complete scan.

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