The association between undetected small-for-gestational age and abnormal admission cardiotocography: A registry-based study

Abstract

Objective: To assess the association between undetected small-for-gestational age (SGA) fetuses and abnormal admission cardiotocography (admCTG) in a low-risk population. Design: An observational study. Setting: Four hospitals in Stockholm-Gotland, Sweden. Sample: A cohort of 127 461 deliveries between 1 February 2012 and 15 June 2020. Methods: This cohort was linked to the Swedish Neonatal Quality Register. Pregnancies were designated as high or low risk at the time of admission to the labour ward according to pre-defined risk measures. SGA was defined as a birthweight at or below the tenth centile and at or below the third centile for gestational age. Main outcome measures: The main outcome was the proportion of undetected SGA by admCTG (normal or abnormal). The secondary outcome was a composite severe adverse neonatal outcome for fetuses born less than 6 hours after admission (Apgar score <4 at 5 minutes, hypoxic–ischaemic encephalopathy grade of 2–3, neonatal seizures and neonatal death). Results: The rate of abnormal admCTG was 4.9%. The proportion of SGA at or below the tenth centile was higher in the abnormal admCTG group than in the normal admCTG group, 18.6% versus 9.7% (odds ratio 2.1, 95% CI 1.9–2.3). Abnormal admCTG and SGA (≤10th) was associated with a more than 20-fold increased risk of an adverse outcome compared with normal admCTG and non-SGA (adjusted odds ratio 23.7, 95% CI 9.8–57.3). The latter had a risk of 1/2000 of an adverse outcome. Conclusions: In this low-risk population, undetected SGA fetuses were more prone to having abnormal admCTG and had a substantially higher risk of severe adverse neonatal outcomes.