More than half of pregnant women experience nausea and vomiting, commonly referred to as morning sickness, according to the American Pregnancy Association. This condition usually begins around the sixth week of pregnancy and begins to dissipate towards the beginning of the second trimester. Although the exact reason morning sickness occurs has not be proven, there are a few likely causes.
Pregnancy Hormones
Pregnancy causes an increase in hormones, especially during the first trimester. The American Pregnancy Association speculates that the pregnancy hormone human chorionic gonadotropin (hCG), in particular, may contribute to morning sickness. HCG is at its highest levels during the time that morning sickness is usually at its peak for many women, explains BabyCenter.com, and pregnant women carrying twins or who have particularly high levels of hCG also tend to have more severe morning sickness. However, other hormonal surges during pregnancy, particularly the hormones estrogen and progesterone, may also come into play.
Sensitivity to Odors
Pregnant women experience an increased sensitivity to smells, and being around certain strong or pungent odors may trigger a bout of morning sickness, suggests BabyCenter.com. The American Pregnancy Association recommends avoiding strong smells and recruiting family members or friends to do the cooking until feelings of morning sickness subside.
Sensitive Digestive Tract
During pregnancy, a woman's digestive tract can slow down or become more sensitive. This can contribute to feelings of morning sickness, but also to other gastrointestinal issues like constipation, bloating, indigestion and heartburn. Although it has not been proven conclusively, BabyCenter.com suggests that women with a type of stomach bacteria called Helicobacter pylori may be more susceptible to morning sickness.
Evolution
The article "Causes of Morning Sickness Revealed" on MSNBC.com suggests that evolution may be responsible for the occurrence of morning sickness. Humans are the only species of mammals that seem to experience morning sickness. Because humans have such a broad diet, and are thus exposed to more things that could potentially harm the fetus, morning sickness evolved to limit a mother's inclination to consume these potentially harmful items. The fact that morning sickness can be triggered by smells of alcohol and cigarette smoke may support this theory, as does the fact that morning sickness tends to peak at the time when the embryonic organ development of the fetus is most susceptible to chemical disruption.
