Nausea and vomiting in early pregnancy: its role in placental development

Nausea and emesis in early pregnancy is a common phenomenon affecting between 50% and 70% of pregnant women, but little is known about the etiology and possible function of this common and often incapacitating condition. Morning sickness has been reported to have a positive effect on pregnancy outcome and is associated with a decreased risk of miscarriage, preterm birth, low birth weight (LBW), and perinatal death. Both human and animal studies have shown that reduced energy intakes in early pregnancy are associated with increased placental weight. Based on evidence from the literature, a hypothesis is proposed that suggests a functional role for the nausea and emesis of pregnancy in stimulating early placental growth. It is suggested that morning sickness, resulting from secretion of hCG and thyroxine, reduces maternal energy intake. As a result, maternal levels of the anabolic hormones, insulin, and insulin growth factor-1 (IGF-1) are lowered. By suppressing maternal tissue synthesis in early pregnancy, we propose that nausea and vomiting in pregnancy helps ensure that nutrient partitioning favors the developing placenta. Evidence is also presented that suggests there may be a positive relationship between morning sickness and preconceptional body mass index (BMI), such that women who are underweight will experience less severe symptoms of morning sickness compared with women with normal preconceptional BMIs.