Suspecting a large baby at term drives intervention without helping the baby

Design

Retrospective cohort, single tertiary centre

Population

21,743 singleton pregnancies with routine 35–37 week ultrasound

Primary outcome

Screening accuracy, mode of delivery, composite adverse outcomes

Effect

Sensitivity 34.9%; intrapartum caesarean aRR 1.47 (1.30 to 1.67) if screen-positive

This retrospective cohort, published in April, followed 21,743 singleton pregnancies scanned routinely at 35–37 weeks at a London tertiary centre. An estimated fetal weight at or above the 90th centile counted as screen-positive, and clinicians knew the result.

The scan detected only 34.9% of large-for-gestational-age babies and 35.6% of macrosomic babies, though specificity was about 96–97%. Screen-positive women were less likely to attempt labour (aRR 0.87) and more likely to have an intrapartum caesarean (aRR 1.47). Missed large babies — the false negatives — had fewer inductions and caesareans than correctly identified ones, with no increase in adverse neonatal outcomes (aRR 0.77, 95% CI 0.56 to 1.06). Women wrongly flagged as carrying a large baby had more operative vaginal deliveries, more caesareans and more maternal morbidity, again with no neonatal difference.

The pattern suggests a labelling effect: the suspicion of a large baby, not the baby's size, drove much of the intervention. It is an argument for targeted rather than universal term screening for fetal size, and for caution in acting on an estimated weight alone.

• Tell women a term scan misses about two in three large babies and often overestimates weight.
• Do not offer induction or caesarean on a suspected large baby alone without other indications.
• Missed large babies in this cohort did no worse, while wrongly flagged mothers had more intervention.
• Reserve term fetal-size scanning for women with a clear indication, such as diabetes.

Why it matters

Challenges the assumption that finding a big baby before labour makes birth safer.

Don't overread it

This is observational and from one centre; it cannot prove screening itself caused the extra intervention.

The statistics, in plain English

Sensitivity of about 35% means the scan finds roughly one large baby in three. An adjusted relative risk of 1.47 for intrapartum caesarean means about half as many again among screen-positive women. The neonatal confidence interval for missed babies (0.56 to 1.06) includes no difference, so missing them did not measurably harm them.

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