Pregnancy sickness as an adaptation to concealed ovulation.

The hypothesis that pregnancy sickness has evolved to expel dietary toxins (PROFET, 1988; 1992; HOOK, 1976) is critically reviewed. Instead it is proposed that pregnancy sickness and mastalgia have evolved to reduce frequency of sexual intercourse in early pregnancy. Reduction in coital frequency during the first trimester has often been documented. Uterine motility during sexual excitement contrasts with lack of spontaneous myometrial activity in the first trimester, except when uterine cramping sometimes occurs as a result of intercourse. Such cramping would be expected to increase the chances of spontaneous abortion.