Outcomes of post-term births: the role of fetal growth restriction and malformations

Abstract

Objective: To evaluate the risks of adverse pregnancy outcomes among term and post-term small for gestational age (SGA) and appropriate for gestational age (AGA) births, before and after excluding infants with congenital malformations.

Methods: We did a population-based study of 510,029 singleton term (37-41 completed weeks) and post-term (at or after 42 weeks) births recorded in the Swedish Birth Register. Odds ratios (ORs) and 95% confidence intervals (CIs) were used to estimate the risks of stillbirth, infant death, convulsions, meconium aspiration, and Apgar score less than 4 at 5 minutes.

Results: Among term births, 2.2% were SGA; among post-term births, 3.8% were SGA. Compared with term AGA births, term SGA births were at increased risk of stillbirth (OR 8.02; 95% CI 6.57, 9.80) and infant death (OR 7.57; 95% CI 6.39, 8.96). Among post-term SGA births, the ORs were 10.56 (95% CI 6.95, 16.05) for stillbirth and 5.00 (95% CI 3.04, 8.22) for infant death. When births with congenital malformations were excluded, the risk of infant death decreased considerably. Risks of convulsions and Apgar score less than 4 were higher in SGA than AGA infants. Post-term AGA infants had no significant increase in the risks of stillbirth or infant death but did have increased risks of convulsions, meconium aspiration, and Apgar score less than 4.

Conclusion: The increased risk of stillbirth in post-term pregnancies is partly explained by an increased rate of SGA infants. The increased risk of death among SGA infants is caused to a large extent by congenital malformations.