An adaptive significance of morning sickness? Trivers-Willard and Hyperemesis Gravidarum

•Morning sickness is common in early pregnancy but is of unknown aetiology.•Its severe form, Hyperemesis Gravidarum (HG), requires hospital admission.•Using Swedish register data, we show that HG is more common among low education mothers.•We confirm a strong girl bias among live births following HG in pregnancy.•HG may be a proximate mechanism for the so called Trivers-Willard hypothesis.

Nausea during pregnancy, with or without vomiting, is a common early indication of pregnancy in humans. The severe form, Hyperemesis Gravidarum (HG), can be fatal. The aetiology of HG is unknown. We propose that HG may be a proximate mechanism for the Trivers-Willard (T-W) evolutionary hypothesis that mothers in poor condition should favor daughters. Using Swedish linked registry data, 1987-2005, we analyze all pregnancies that resulted in an HG admission and/or a live birth, 1.65 million pregnancies in all. Consistent with the T-W hypothesis, we find that: (i) HG is associated with poor maternal condition as proxied by low education; (ii) HG in the first two months of pregnancy is associated with a 7% point increase in live girl births; and (iii) HG affected pregnancies have a 34-percent average rate of inferred pregnancy loss, higher among less educated women.