27.02.6 Nausea

Nausea and vomiting during pregnancy (also known as morning sickness) is common during the early stages of pregnancy. These are multiple causes ranging from changes in circulating steroid hormones such as oestrogen and progesterone to hypoglycaemia resulting from placental transfer of glucose from the mother to the foetus. In most cases no treatment is required as the condition will normally disappear after the first trimester of pregnancy. However, severe morning sickness known as hyperemesis gravidarum (HG) affecting around one in 1,000 pregnant women can occur. The symptoms of hyperemesis gravidarum include repeated vomiting, weight loss and dehydration. Treatment usually involves hospitalisation, and the administering of intravenous liquids and nutrition. The possible complications associated with untreated hyperemesis gravidarum include electrolyte imbalances, extreme depression and anxiety and malnourishment of the foetus.

In extreme cases of morning sickness, antiemetics may be prescribed. These drugs work by blocking neurotransmitter inputs to the chemoreceptor trigger zone (CTZ), the principal vomiting control centre, of the medulla. First choice is ondansetron, a blocker of 5HT-3 receptors found in the CTZ, given orally or by injection. This drug, which is also used as an antiemetic during cancer chemotherapy, has a number of unwanted effects such as headache and GI upset. Other drugs used include promethazine (histamine H1 receptor blocker) and metoclopramide and chlorpromazine (dopamine D2 receptor blockers).