Morning sickness is experienced by more than half of all pregnant women and has long been considered one of the unfortunate side effects of pregnancy. Women all over the world hate morning sickness and many take different measures to reduce its effects.
However, morning sickness may be a defense mechanism to protect the unborn child. Numerous research has suggested that nausea and vomiting is triggered purposefully to expel potentially harmful foods such as meat, which would have contained parasites in years gone by, and strong tasting vegetables that may contain plant toxins. These materials could harm the baby at a crucial stage when its organs are forming.
The nausea and vomiting reaction would then deter the mother-to-be from consuming that item again. Likewise, tea and coffee suddenly smell disgusting to a pregnant woman, even if she normally likes them.
This may be to protect her baby from the negative effects of caffeine, which increases the risk of miscarriage. The same effect is sometimes seen with cigarette fumes.
So morning sickness directs the expectant mother into a healthy lifestyle, which she might not otherwise adopt.
Another theory about morning sickness suggests that it may be a side effect of the mother’s newly immune-compromised state. All pregnant women have a certain degree of immune suppression because half of the baby’s genes come from the father and are foreign to her, therefore, the immune system has to be restricted in order to prevent her from rejecting it as a foreign body.
Scientists also hypothesize that there is a constant physiological battle between the baby and the mother over how nutritional resources are divided between them. The baby needs enough resources to continue to develop, while the mother needs to be healthy in order to carry the pregnancy to term.
A side effect of this internal battle is thought to be morning sickness. This theory is called "gestational conflict."
The problem with this theory is that we are the only placental mammals that experiences morning sickness during pregnancy.