Reproductive Epidemiology in an Evolutionary Perspective: Why Bigger May Not Be Better

Abstract

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.

Implications of Using a Fetuses‐at‐Risk Approach When Fetuses Are Not at Risk

“…Conventional rates among the unexposed infants in this unfeasible experiment would additionally provide a reasonable estimate of the 'pure' effect of immaturity. 17,18,26 The FAR approach has been presented as a causal framework for perinatal epidemiology 5,8,23 and, in a broad sense, fetuses can be considered as being at risk of a range of postnatal endpoints, although they can only experience these outcomes if they are born alive (and birth itself poses its own dangers to the infant 13,27 ). Given their strong dependence on the pattern of live birth, rates of postnatal outcomes based on a FAR analysis have no clear-cut causal interpretation.…”

Reproduction after the loss of a child: a population-based matched cohort study

“…Several studies show that the ability to conceive and carry a pregnancy to term may be reduced as a result of stress (Buck Louis, Lum, Sundaram, Chen, Kim, Lynch, Schisterman and Pyper, 2011, László, et al, 2013, Lynch, Sundaram, Maisog, Sweeney and Buck Louis, 2014, Nepomnaschy, Sheiner, Mastorakos and Arck, 2007, Ramezanzadeh, Noorbala, Abedinia, Rahimi Forooshani and Naghizadeh, 2011, Wisborg, et al, 2008; however, our study did not include a direct measure of stress or of fecundability, given that we did not know which women tried to have another child. Given that loss of an infant has been a very common event throughout human history (Basso, 2014), it is possible that individuals who could conceive again shortly after the loss of a child may have been selected.…”

Evolutionary justifications for human reproductive limitations

“…Early effects, e.g., prior to the recognition of pregnancy, or conception failures, minimize maternal investments, while later events include situations where Bmothers, whose very survival is threatened by pregnancy and childbirth, appear to play a crucial role in determining length of gestation and size of baby. Having smaller babies may thus have conferred an advantage to mothers, despite the increased risk to individual offspring^ [30].…”