One of the most familiar symptoms associated with pregnancy is the combination of nausea and vomiting that is commonly called “morning sickness.” Not all pregnant women will experience this discomfort, but for the 75 to 80 percent of women who do, it is probably one of the first things that lead them to suspect they may be pregnant.
“Morning sickness” is not in fact an entirely appropriate name for this condition, since it is not actually confined to the morning for many women. A woman who has never been through pregnancy before may be alarmed if she is still feeling these symptoms later in the day, but it is actually quite normal and no cause for undue concern—though some disappointment is understandable. The medical term for the condition is “emesis gravidarum,” and some experts prefer the name “Nausea and Vomiting of Pregnancy,” or NVP, rather than morning sickness.
Pregnancy-related nausea begins for most women from two to eight weeks after conception—often even before a menstrual period has been missed. Perhaps surprisingly given how common it is, the physical causes of pregnancy nausea are not entirely understood. There is however a good deal of evidence pointing to the influence of hormone levels in the mother’s body as a primary cause. The way these hormones function helps explain why pregnancy nausea generally occurs when it does and why for most people it ends when it does.
During a woman’s monthly reproductive cycle, her body increases the level of estrogen and progesterone production for a few peak days during ovulation to make the uterus more receptive to fertilization. If conception does occur, the developing embryo immediately begins to secrete a new hormone—human chorionic gonadotropin or hCG. HCG signals the mother’s body to continue producing extra estrogen and progesterone that is now needed to support the embryo, instead of letting their levels drop as they usually do during the menstrual cycle. The production of hCG is one of the very first physical effects of pregnancy. It can be detected in the blood serum within 24 to 48 hours and in the urine just a few days later. Most early pregnancy tests in fact work by identifying hCG in the woman’s blood or urine.
During these first few weeks of pregnancy, the placenta is just starting to develop and is not yet capable of protecting and sustaining the developing embryo. At this time, it is essential for the hCG hormone to stimulate the mother’s body to maintain elevated levels of estrogen and progesterone. If progesterone levels drop too low, for instance, spontaneous abortion is likely to occur. HCG levels continue to rise and then peak at approximately 8 weeks. By the 12-week point, the placenta should have developed sufficiently to start producing these hormones itself. The levels of hCG will begin to drop gradually, and for many women morning sickness will lessen or cease, though hCG does not disappear completely until after delivery.
The appearance of hCG in early pregnancy is considered the most likely cause of pregnancy nausea. Because hCG plays such a crucial role in the early development of the fetus, early pregnancy nausea can actually be a positive sign that the pregnancy is progressing normally.
While it’s not entirely certain that hCG triggers pregnancy-related nausea directly, there are a number of coincidences that suggest it plays a significant role. For the majority of women, morning sickness is most common and severe during exactly the times when hCG is at its highest level—from the 4th to the 12th weeks of pregnancy. HCG levels are significantly higher in women carrying twins or multiples, and these women are also much more prone to pregnancy nausea. Higher levels of hCG and pregnancy nausea are also found together in women carrying female fetuses. Women who smoke have lower levels of both hCG and morning sickness.
HCG also contributes to pregnancy nausea indirectly, because of its influence in causing estrogen levels to increase. Estrogen fluctuation has long been recognized as a cause of nausea, vomiting and diarrhea for many women during their menstrual periods.
In summary, while not all women will experience pregnancy-related nausea (morning sickness), if they do it is most likely to start early—sometimes within a week or two of conception—and to decrease or disappear toward the end of the first trimester, though it is not uncommon or abnormal for it to continue longer in a significant number of cases.
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