Nausea and vomiting, among the most unpleasant aspects of early pregnancy, affects around 75 percent of all pregnant women during the first three months of pregnancy, according to BabyCenter. Human chorionic gonadotropin, or hCG levels in the blood also rise in early pregnancy, suggesting a correlation between the two.
Human chorionic gonadotropin is a hormone produced by the layer of the embryo that develops into the placenta. Pregnancy tests that use either urine or blood also measure levels of a subunit of hCG called beta hCG. In a non-pregnant woman, serum beta hCG levels are normally less than 5 mIU/ml. Beta hCG levels double every 48 to 72 hours in a normally developing pregnancy. Multiple gestations and hydatidiform mole, an abnormal pregnancy where just the placental tissue grows, have higher than normal beta hCG levels.
Small amounts of hCG can be detected in the blood as early as 24 to 48 hours after the embryo implants into the uterine wall, with urine detection usually following several days later, RnCeus explains, which explains why some women feel queasy starting as early as the first missed period. As hCG levels rise, nausea in pregnancy normally worsens. Levels of hCG normally peak around eight weeks, then decline over the next eight weeks. Nausea and vomiting of pregnancy peak around nine to 10 weeks, then gradually disappear in most women by 14 to 16 weeks, obstetrician Dr. Marjorie Greenfield explains on the Dr. Spock website.
Women with pregnancies that produce higher levels of hCG, such as twin pregnancies, generally experience more nausea and vomiting in pregnancy. These pregnancies also produce higher than normal amounts of hCG, because of the increased amount of trophoblast tissue, RnCeus states. Conversely, women whose pregnancies end in miscarriage often have fewer pregnancy symptoms, including less nausea and vomiting, Greenfield reports. A lack of nausea and vomiting does not, however, mean that a pregnancy will end in miscarriage, since a small number of women never experience nausea or vomiting in pregnancy.
While the correlation between hCG and nausea makes sense, other factors may also increase the chance of developing morning sickness, which, contrary to its name, doesn't limit itself to morning hours. Rising estrogen levels, an increased sense of smell and stress may also increase the chance of developing nausea or vomiting.
Rising hCG levels in pregnancy indicate a normally developing pregnancy. While understanding that rising hCG may also cause increased nausea or vomiting doesn't make the experience less unpleasant, knowing that they indicate a healthy pregnancy helps some women deal with the temporary discomfort.