Pregnancy sickness and embryo quality

Nausea and vomiting are routine features of early pregnancy in humans. But are they adaptive or pathological? Several lines of evidence suggest that they protect mothers and developing embryos from dietary mutagens and pathogens. Nausea and vomiting in pregnancy (NVP) coincide with the vulnerable period of embryogenesis, are associated with food aversions, and are predictors of positive pregnancy outcomes. Here, I argue that NVP is not directly adaptive, but arises as a byproduct of genetic conflict between mother and embryo. The negative correlation between first-trimester spontaneous abortion and NVP is not the result of protection of embryos from environmental mutagens or pathogens, but the result of intrinsic chromosomal defects. These low-quality embryos produce subnormal levels of human chorionic gonadotropin, a hormone involved in pregnancy maintenance, and the probable proximate trigger for NVP.