Morning sickness is a frequent occurrence in approximately 75 percent of all pregnancies. Despite the fact that it is so common, Dr. Rachel Vreeman, a pediatrician with the Indiana University School of Medicine, explains that doctors and scientists don't really know what causes it. The best they've been able to do is make educated guesses.
Human Chorionic Gonadotropin (HCG)
HCG is a hormone released after the embryo implants into the lining of the uterus and begins to grow. It is the hormone detected in both home and blood pregnancy tests. BabyCenter.com notes that the presence of HCG may be to blame for morning sickness, since the uncomfortable symptoms coincide with the peak of HCG during pregnancy.
Estrogen and Progesterone
The hormones estrogen and progesterone increase to help maintain the pregnancy. Though the exact reason is unknown, these hormonal shifts are thought to be a possible cause of nausea during early pregnancy.
Increased Sense of Smell
Pregnant women often experience a heightened sense of smell in the early weeks of pregnancy. Smells that previously were easily tolerated become offensive and can easily trigger nausea. The smell of some perfumes, foods and other odors commonly encountered during the day can cause this unpleasant reaction.
Fetal Protection
According to MSNBC, scientists believe that morning sickness may be a natural response by the mother's body to protect her unborn baby from possibly dangerous foods and substances. They theorize that morning sickness most often occurs during the first trimester when the baby's internal organs and systems are forming. By creating a response such as nausea, pregnant women often avoid certain foods, alcohol and cigarette smoke, thus protecting the unborn baby.
Medical Conditions
Medical conditions occurring during pregnancy may cause nausea and vomiting, but are completely unrelated to the pregnancy. MayoClinic.com notes that diseases of the thyroid and liver can cause these symptoms, though nausea and vomiting associated with these causes tend to last longer and be more severe than typical morning sickness.
Other Possible Causes
Other factors may contribute to morning sickness. Stress, a slowed digestive system, carrying more than one baby and fatigue are all thought to cause morning sickness as well.
