The association between the indication for antenatal fetal surveillance and an abnormal result: A retrospective cohort study

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Objective: Indications for antenatal testing have expanded, but their impact on abnormal test prevalence is unclear. We evaluated factors associated with non-reactive non-stress tests (NSTs) and abnormal biophysical profiles (BPPs) in high-risk pregnancies. Study design: In this retrospective cohort, we identified all singleton pregnancies that underwent serial antenatal testing with an NST ≥ 28 weeks at four testing sites in an academic health system between January 2022 and May 2023. A BPP was reflexively performed if the NST was non-reactive. The primary outcome was prevalence of a BPP score <8/10, with the rate of NST non-reactivity as the secondary outcome. A stepwise Generalized Estimating Equation (GEE) model was used to identify indications associated with these outcomes after adjusting for race, parity, gestational age (GA), and testing site. Results: Among 19,034 NSTs performed in 2695 patients (mean 7.1 NSTs/patient), only 84 (0.4%) failed to provide a reassuring result (i.e., reactive NST and/or BPP ≥ 8), while 99.6% (18950/19034) had a passing result. The overall NST reactivity rate was 87.3%. Of the 2327 BPPs performed for a non-reactive NST, 2243 (96.4%) yielded a passing score of ≥8/10. Several testing indications were significantly associated with NST non-reactivity, including chronic hypertension (aOR, 1.5; CI, 1.2-1.8), fetal growth restriction (FGR) (aOR, 1.4; CI, 1.1-1.6), other fetal/placental complications (aOR, 1.7; CI, 1.2-2.3), obesity, (aOR, 1.3; CI, 1.1-1.5) and hypertensive disorders of pregnancy (HDP) (aOR, 1.3; CI, 1.0-1.6). However, only FGR (aOR, 2.7; CI, 1.5-4.7), vaginal bleeding (aOR, 6.0; CI, 1.4-25), and post-term (aOR, 8.6; CI, 1.4-51.6) were significantly associated with BPP < 8. For patients in testing for a single indication (79.4% of patients and 73.1% of NSTs), only chronic hypertension (aOR, 1.5; CI, 1.1-2.1) was significantly associated with a higher odds of non-reactivity, while only post-term was associated with BPP < 8 (aOR, 7.2; CI, 1.4-35.6). Conclusions: Our results demonstrate that in the modern-day clinical environment, only a small fraction (<1%) of fetal tests yield an abnormal result. Several testing indications were more likely to have a non-reactive NST, but only FGR, vaginal bleeding, and post-term were more likely to also have an abnormal BPP score < 8. As presenting for serial fetal testing remains a significant barrier for many patients, these data can support data-informed patient counseling and help clinicians prioritize those at greatest risk. Further research should target investigating downstream perinatal outcomes after abnormal antenatal testing and how changes in testing recommendations impact these outcomes.

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