Pregnancy sickness is widespread yet its etiology is poorly understood. It is almost certainly endocrine in origin and most likely a product of placental hormones, with human chorionic gonadotropin being the strongest candidate. It has long been known that greater levels of nausea and vomiting during pregnancy are associated with a lower incidence of spontaneous abortion, yet the causal mechanisms remain unclear. One current popular explanation is that nausea and vomiting during pregnancy is fetoprotective, inducing aversions to foods, especially meat, dairy and seafoods, which may carry toxins, pathogens or mutagens. However, most spontaneous abortions arise from genetic or epigenetic defects that are present at or near conception. Moreover, measurements of human chorionic gonadotropin (hCG) at the time of implantation, particularly its hyperglycosylated isoform, accurately predict subsequent spontaneous abortion. Thus the developmental fate of most embryos is fixed before the onset of the symptoms of pregnancy sickness. An alternative explanation for the link between pregnancy sicicness and spontaneous abortion is the embryo quality hypothesis: high quality embryos are both more likely to produce the biochemical antecedents of pregnancy sickness and avoid spontaneous abortion. Recent work has shown that the link between pregnancy sickness and spontaneous abortion grows stronger with maternal age, dramatically so in mothers 35 or older. This reflects the parallel rise in the incidence of autosomal aneuploidies with maternal age. The link between pregnancy sickness and spontaneous abortion exists not because nausea and vomiting during pregnancy is fetoprotective, but because nausea and vomiting is an index of a high quality embryo. Pregnancy sickness is not adaptive per se, but the result of an antagonistic pleiotropy over thyroid function, where embryos use hCG to modulate maternal thyroid hormone production during gestation. Embryos benefit from the thyroid hormone production that is key to neurodevelopment, but produce maternal nausea and vomiting as a by-product. Pregnancy sickness, however, may still serve to protect embryo quality but by a different mechanism that posited under the MEPH. Embryo quality is protected by calibrating the dietary intake of a micronutrient - iodine - critical to neuromotor development. For most humans over most of our evolutionary history, iodine has been in short supply, and iodine deficiency is still the most common source of cognitive impairment across the globe. Thus it is of interest that the food aversions most commonly associated with pregnancy sickness, to meat, dairy and seafoods, are also the chief dietary sources of iodine. There is a further intriguing property about iodine: both too little and too much during early pregnancy are damaging to embryo brain development. Given that pregnancy sickness is closely linked to iodine intake and thyroid function (hypothyroidism is associated with lower levels of nausea and vomiting, hyperthyroidism with more), an obvious interpretation emerges. The previously described link between diet and pregnancy sickness - pregnancy sickness is less likely when plants and particularly corn/maize are the sole food staples - arises not because plant food staples are safe, as previously suggested, but because these foods are iodine poor and may, in addition, be goitrogenic. Pregnancy sickness, which reduces the dietary intake of iodine, is clearly maladaptive under conditions of iodine deficiency and hypothyroidism. Conversely, higher levels of pregnancy sickness induced by hyperthyroidism may protect embryos from the inimical effects of excessive dietary iodine during early gestation by reducing the intake of iodine rich foods. (C) 2016 Elsevier Inc. All rights reserved.