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The time interval between pregnancies is viewed as an important and modifiable risk factor for adverse birth outcomes. The incidences of preterm birth, small for gestational age birth, and low birth weight have each been repeatedly shown to follow a strong J-shaped relation to the time interval between pregnancies. Typically, short intervals (less than 18 months between previous birth and subsequent conception) and long intervals (more than 23 months) have a higher risk of these birth outcomes compared with intermediate intervals of 18-23 months.
In this research the authors set out 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.
