Objective: To estimate the impact of body mass index (BMI) categories on duration of the nonstress test (NST), and the need for additional tests of fetal wellbeing or interventions.
Methods: We conducted a retrospective cohort study of women with singleton pregnancies who had an NST for the indications of severe obesity (BMI ≥40 kg/m2), diabetes (pre-gestational or gestational), hypertensive disorders (chronic, gestational, and preeclampsia), and others between 1 January 2015 until 31 December 2016. NST durations (<30 and ≥30 min) were compared between groups first based on BMI (kg/m2) categories (<30, 30-39, 40-49, and ≥50) and then based on BMI and comorbidities: (1) severe obesity alone, (2) severe obesity and comorbidities, and (3) comorbidities alone. We compared the results of the NSTs, any subsequent fetal testing or interventions, and pregnancy outcomes among groups. Demographic information was compared using t-tests for continuous data and χ2 analyses or Fisher's exact test, if the cells sizes were small, for categorical data. NST durations based on BMI groups, as well as BMI and comorbidities groups, were compared using linear mixed models and ANOVA.
Results: Three hundred and fifty-one women underwent 1665 NSTs during the study period. After excluding women <18 and >50 years, gestational age <30 weeks, fetal anomalies, and NSTs lasting longer than 60 min, the study population included 313 women and 1471 NSTs. The mean NST duration in minutes of the BMI ≥50 (32.2 ± 9.6) category was significantly longer than the NST duration of the other BMI categories (BMI <30: 29.4 ± 8.3; BMI 30-39: 29.7 ± 8.9; BMI 40-49: 29.9 ± 8.3) (p=.05). Additionally, there was an increased percentage of NSTs lasting ≥30 min as the BMI category increased (p=.005). Women in the severe obesity and comorbidities group (n = 79) were less likely to have a reactive NST than women with severe obesity alone (n = 56) or comorbidities alone (n = 178) (92% vs. 97% vs. 98%, p<.0001). They were also more likely to need a biophysical profile (BPP) (8% vs. 3% vs. 2%, p<.0001). Of the 25 women that were sent to the labor and delivery unit for evaluation, 20 (80%) were admitted and delivered. The reasons for delivery were hypertension (n = 9, 45%), an abnormal NST (n = 5, 25%), oligohydramnios (n = 4, 20%), and labor (n = 2, 10%).
Conclusions: We found an increase in NST duration as the BMI increased. Women with severe obesity and a comorbidity were more likely to have a nonreactive NST and require a subsequent BPP. Of the 20 patients delivered due to NST surveillance, the most common indication for delivery was hypertension.
Keywords: BMI; Morbid obesity; NST; antepartum fetal testing; nonstress test; severe obesity.