A new study published in Obstetrics and Gynecology examined associations among interpregnancy interval the duration from the preceding birth to the conception of the next born index child and adverse birth outcomes. This prospective cohort study utilized population based Swedish registries from 1973 through 2009 to estimate the associations between interpregnancy interval referent 18 23 months and adverse birth outcomes preterm birth less than 37 weeks of gestation low birth weight LBW less than 2 500 g and small for gestational age SGA greater than 2 SDs below average weight for gestational age . The results showed higher chances of preterm birth and LBW after a short interpregnancy interval 0 5 months compared to offspring born after an interpregnancy interval of 18 23 months. Except for preterm birth associations were attenuated in cousin comparisons. A small association between a short interpregnancy interval and preterm birth persisted in sibling comparisons but associations with LBW and SGA were variable. For pregnancy intervals of 60 months or more the probability of preterm birth LBW and SGA were also higher when compared to the reference interval. Meanwhile associations between long interpregnancy interval and adverse birth outcomes remained through cousin and sibling comparisons. Furthermore post birth interval analyses showed familial confounding existed for short interpregnancy intervals but supported independent associations for long interpregnancy intervals. Hence it was inferred that familial confounding explains most of the association between a short interpregnancy interval and adverse birth outcomes whereas associations with long interpregnancy intervals were independent of measured and unmeasured factors.