Fetal size and growth velocity in the prediction of intrapartum caesarean section for fetal distress

OBJECTIVE: To evaluate and compare third trimester ultrasound measurements of fetal size and growth velocity in the prediction of intrapartum operative delivery for fetal distress and admission to the special care baby unit in a low risk antenatal population undergoing labour at term. DESIGN: Retrospective analysis of prospectively collected ultrasound data. SETTING: Ninewells Hospital, Dundee, Scotland. POPULATION: Three hundred and ninety-eight women previously enrolled in a longitudinal study of intrauterine volume undergoing labour at a gestational age of > 37 weeks. METHOD: Fetal abdominal area (FAA) standard deviation scores (Z scores) were calculated for size at 32 and 36 weeks of gestation together with the growth velocity Z scores between these two gestational ages. Receiver-operator characteristics were calculated for fetal abdominal area Z scores at 32, 36 weeks and velocity Z scores in the prediction of caesarean section for fetal distress and/or admission to the special care baby unit. MAIN OUTCOME MEASURES: Intrapartum caesarean section for fetal distress and admission to the special care baby unit with a diagnosis of perinatal hypoxia. RESULTS: Pregnancies ending in caesarean section for fetal distress or admission to the special care baby unit (n = 17) had significantly lower fetal abdominal area Z scores at 36 weeks of gestation (mean Z score -0.71 vs -0.18) and lower fetal abdominal area growth velocity (mean Z score - 1.31 vs -0.01). Taking a cutoff Z score of -1.2 derived from the receiver-operator characteristic curve, fetal abdominal area velocity has a sensitivity of 65% and specificity 75% for caesarean section for fetal distress and/or admission to the special care baby unit. CONCLUSION: Growth velocity of the fetal abdominal area in the third trimester is superior to a single measurement of the fetal abdominal area at either a mean of 32 or 36 weeks of gestation in the prediction of caesarean section for fetal distress and admission to the special care baby unit in low-risk women labouring at term. These results support the hypothesis that in the third trimester at least, growth rate in utero is more relevant to intrapartum performance and immediate perinatal outcome than estimates of fetal size alone.