What is the Ideal Spacing Between Pregnancies?

MedicalResearch.com Interview with:
Steven Ball
Telethon Kids Institute
University of Western Australia
West Perth, WA 6872, Australia

Medical Research: What are the main findings of the study?

Answer: Our study suggests that the amount of time between pregnancies has less of an effect on birth outcomes than previously thought.

Relative to pregnancies that started 18-23 months after a previous birth, pregnancies that followed shorter spacing had very little increased risk of preterm birth, low birth weight or small-for-gestational-age. Longer pregnancy spacing showed increased risk of low birth weight and small-for-gestational-age, but not of preterm birth.

Medical Research: Were any of the findings unexpected?

Answer: Our results challenge existing thinking on the effects of pregnancy spacing.

Many studies have shown that pregnancies that start within two years of a previous birth, or more than five years after a previous birth have a higher incidence of preterm birth, low birth weight and small-for-gestational-age. Until now the science on pregnancy spacing has been based on comparing different mothers. While researchers try to adjust for maternal risk factors, there’s always the possibility that key aspects of differences between mothers are overlooked or not measured effectively.

Using data from more than 40,000 Western Australian mothers who each had three or more pregnancies, we asked whether a mother’s change in pregnancy interval changes the risk of adverse birth outcomes. This approach treats each mother as her own control for factors that predispose her children to adverse birth outcomes.

Because we found weak effects of short pregnancy spacing, our results question the causal effect of short pregnancy spacing on adverse birth outcomes. The results for long pregnancy spacing were less consistent, with no apparent effect for preterm birth, but strong effects of increased risk for low birth weight and small-for-gestational-age.

This suggests that what appears to be an effect of short (and long) pregnancy interval in observational studies which compare women with different pregnancy intervals is due to factors other than pregnancy interval which these studies do not measure. The methods used in our study, of comparing births within individual mothers, could be used for other factors when randomised controlled trials are not possible.

Medical Research: What should clinicians and patients take away from your report?

Answer: Based on our results, mothers who fall pregnant again within two years should be reassured about the effects of the short interval. However this is the first study to look at pregnancy spacing in this way, and it is important to be aware that the World Health Organization recommends mothers wait two years between the birth of one child and starting to try for the next.

Short pregnancy spacing is a useful predictor of risk, even if it isn’t a causal factor. We therefore recommend that clinicians continue to treat short pregnancy spacing as a useful flag of increased risk of adverse birth outcomes, but remain vigilant for maternal risk factors that may accompany a short time between pregnancies.

Medical Research: What recommendations do you have for future research as a result of this study?

Answer: To our knowledge, our study is the first to apply within-mother methods to test for an effect of pregnancy spacing on adverse birth outcomes. Further research using similar methods in other populations is needed to see how generalizable our results are. Women in developing regions may have slower recovery between pregnancies due to poor access to nutrient rich foods. Furthermore, it would be useful to apply our method to other health outcomes where pregnancy spacing may have an effect, such as maternal health, birth defects and child mental health.

Ball Stephen J, Pereira Gavin, Jacoby Peter, de Klerk Nicholas, Stanley Fiona J. Re-evaluation of link between interpregnancy interval and adverse birth outcomes: retrospective cohort study matching two intervals per mother
BMJ 2014; 349:g4333