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.