Pregnancy and childbirth have been shaped by natural selection, and some reproductive endpoints, such as preterm birth and low birth weight, may be understood differently by considering our history. Mothers, whose very survival is threatened by pregnancy and childbirth, appear to play a crucial role in determining length of gestation and size of the baby. Having smaller babies may thus have conferred an advantage to mothers, despite the increased risk to individual offspring. Furthermore, it is unknown how much of the excess mortality risk of small and preterm babies is due to preterm birth and low birth weight per se, rather than to pathological factors that reduce birth weight and length of gestation. The apparently paradoxical phenomenon of intersecting mortality curves can be explained in this framework, which may shed a different light on these two central entities in perinatal epidemiology.