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.