Authors:
Ball SJ, Pereira G, Jacoby P, de Klerk N, Stanley FJ
Authors notes:
BMJ (Online) 349.
Keywords:
interpregnancy interval, adverse birth outcomes, preterm birth, small for gestational age, low birth weight
Abstract:
Objective: to re-evaluate the causal effect of interpregnancy interval on adverse birth outcomes, on the basis that previous studies relying on between mother comparisons may have inadequately adjusted for confounding by maternal risk factors.
Setting: Perth, Western Australia, 1980-2010.
Participants: 40 441 mothers who each delivered three liveborn singleton neonates.
Main outcome measures: Preterm birth (<37 weeks), small for gestational age birth, and low birth weight (<2500 g).
Results: Within mother analysis of interpregnancy intervals indicated a much weaker effect of short intervals on the odds of preterm birth and low birth weight compared with estimates generated using a traditional between mother analysis.
The traditional unmatched design estimated an adjusted odds ratio for an interpregnancy interval of 0-5 months (relative to the reference category of 18-23 months) of 1.41 for preterm birth, 1.26 for low birth weight, and 0.98 for small for gestational age birth.
In comparison, the matched design showed a much weaker effect of short interpregnancy interval on preterm birth and low birth weight, and the effect for small for gestational age birth remained small.
Both the unmatched and matched models estimated a high odds of small for gestational age birth and low birth weight for long interpregnancy intervals (longer than 59 months), but the estimated effect of long interpregnancy intervals on the odds of preterm birth was much weaker in the matched model than in the unmatched model.
Conclusion: This study questions the causal effect of short interpregnancy intervals on adverse birth outcomes and points to the possibility of unmeasured or inadequately specified maternal factors in previous studies.