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Additional precautions warranted for babies predicted to be at the extremes of birth weight

To reduce adverse events linked to stillbirth, infant death and neonatal morbidity at term, early term delivery or closer surveillance should be considered for infants with predicted birth weight at or below the 25th and at or above the 85th centile, according to data from a recent study that appeared in PLoS Medicine.

“Whether [statistical thresholds outside the expected birth weight for gestational age] optimally define the risk of perinatal mortality and morbidity at term is unknown,” Stamatina Iliodromiti, MD, MRCOG, from the University of Glasgow School of Medicine and Glasgow Royal Infirmary, and colleagues wrote. “There is conflicting evidence whether customized charts perform better than non-customized centiles in predicting adverse perinatal outcome and the strength of evidence for supporting this approach, particularly for term infants, has been challenged.”

Iliodromiti and colleagues performed a population-based cohort study of 979,912 term singleton pregnancies between 1992 and 2010 in Scotland to assess risk for infant death, stillbirth, overall mortality, low Apgar score and neonatal unit admission in non-customized and partially customized birth weight centiles. They calculated optimal thresholds that predicted outcomes for both non- and partially customized birth weight centiles, then compared the prediction of mortality using area under the receiver operator characteristic curve (AUROC) and net reclassification index (NRI). 

The results revealed that birth weights in the 25th centile or less were associated with higher risk for all mortality and morbidity outcomes. The researchers also observed increased risk for stillbirth, low Apgar score and neonatal unit admission from the 85th centile. They saw these similar patterns in magnitude of risk across all five outcomes in both non- and partially customized birth weight centiles. Compared with non-customized birth weight centiles (AUROC = 0.62; 95% CI 0.6-0.63), partially customized birth weight centiles did not improve the discrimination of mortality (AUROC = 0.6; 95% CI, 0.6-0.62). Partially customized birth weight centiles also underperformed in reclassifying pregnancies to different risk categories for both fatal and non-fatal adverse outcomes (NRI –0.027; 95% CI, –0.039 to –0.016]; P < .001).

Researchers also reported that accelerating delivery for an expected birth weight of the 25th noncustomized centile or less, rather than less than the 10th (assuming that the intervention would have 69% effectiveness for mortality), would result in pre-emptive delivery of 159,025 additional women to prevent 377 fatal events (stillbirth and infant deaths). Thus, to prevent one death, 422 (95% CI, 381-468) additional pregnancies below this new threshold would need to be delivered. If the equivalent 25th percentile or less threshold was used for the partially customized centiles, intervention in an additional 463 (95% CI, 417-516) pregnancies would be needed to prevent one fatal event. Using the threshold of the 85th centile or greater for defining large for gestation babies rather than greater than the 90th centile and delivering them earlier, (assuming 69% effectiveness of intervention), would require an additional 721 (95% CI, 598-947) or 826 (95% CI, 638-1,091) pre-emptive deliveries, respectively for noncustomized and partially customized centiles, to prevent one fatal event.

“Despite dramatic improvements in maternal and neonatal care, we showed that even at term (37 to 43 weeks) birth weight remains strongly associated with the risk of stillbirth and infant death, low Apgar score and admission to the neonatal unit, Iliodromiti and colleagues wrote. “Replication of our results in other independent large datasets is warranted. A clinical trial looking at morbidity outcomes (since the small number of mortality outcomes renders a clinical trial with primary outcome stillbirth and infant deaths practically unfeasible) will clarify whether early term intervention at the proposed thresholds of birth weight will be beneficial.” – by Savannah Demko

Disclosure: Iliodromiti is funded by a UK Medical Research Council skills development fellowship. Please see the full study for a list of all other researchers’ relevant financial disclosures.