The main connection between morning sickness and miscarriage is that women who experience nausea and vomiting during pregnancy have a lower risk of pregnancy loss. Additionally, women who experience absolutely no sickness at all have a slightly elevated risk of having a miscarriage when compared to women with severe to moderate nausea. The reason why this is so is not fully understood, as no one knows exactly what causes morning sickness.
There are several theories as to why morning sickness and miscarriage rarely go together. The first is that sickness in pregnancy is sign of hormones increasing readily and causing reactions in the woman’s body. A lack of normal pregnancy symptoms, like nausea, may be a sign that hormonal levels are not increasing as they should. These hormones, primarily progesterone and estrogen, are necessary at preventing menstruation and supporting a healthy pregnancy. A deficit in these hormones may cause the body to not realize it is pregnant and spontaneously abort the pregnancy.
Another theory on morning sickness and miscarriage rates state that nausea and vomiting during pregnancy are nature’s way to ensuring the mother does not eat anything harmful. Vomiting may also help purge the stomach of potentially harmful pathogens. This theory is not highly supported, partly because many times a woman becomes sick at the thought of eating healthy foods while tolerating junk food well.
Whether there is any correlation between the severity of morning sickness and miscarriage is not entirely known. There typically isn’t a set amount of sickness that is considered “healthy” or “unhealthy,” so long as the mother is still able to eat and nourish her body and does not become dehydrated. Aside from that, women experience nausea in pregnancy at varying levels and lengths of time with no apparent link to pregnancy health.
There are plenty of women, however, who experience no morning sickness or nausea at all and go on to have healthy babies. It is possible that if the hormone theory were true, these women may simply respond better to the increase in hormones than most. Another possibility is that these women eat healthier and avoid risky foods and other substances, and may have less of a need for the added protection of vomiting and nausea. It may also be that the baby or placenta produces hormones or enzymes that cause some women to have sickness and others to not.