Short gaps (less than two years) between pregnancies have been shown again and again to be associated with poorer birth outcomes (particularly smaller babies) than medium length gaps (3-5 years), even after controlling for identifiable confounders such as maternal age. This result was replicated by Ball et al., but they made a further analysis in which mothers who had had at least three children acted as their own controls. The association between short birth interval and low birth weight disappears. So it would appear that both events – birth interval and birth weight – are linked by underlying biological factors. The lesson – whenever possible look for differences within person, before comparing the differences across people, and remain very sceptical about cause and effect associations based on cross-sectional risk-adjustment alone.
— Richard Lilford, CLAHRC WM Director
- Ball SJ, Pereira G, Jacoby P, de Klerk N, Stanley FJ. Re-evaluation of link between interpregnancy interval and adverse birth outcomes: retrospective cohort study matching two intervals per mother. BMJ. 2014; 349: g4333.