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Nausea and vomiting in pregnancy - Background information
What causes it?
- The cause of nausea and vomiting in pregnancy is uncertain. Several associated factors may influence its development, but some causative mechanisms have also been suggested:
- Human chorionic gonadotrophin (hCG):
- There is general agreement that the stimulus for nausea and vomiting in pregnancy is of placental origin. The hCG theory is supported by the temporal relationship between nausea and vomiting and hCG levels. In addition, nausea and vomiting are associated with multiple pregnancies and molar pregnancy, both of which involve increased levels of hCG.
- Oestrogen:
- Nausea and vomiting in pregnancy are more common when estradiol levels are increased. When levels are decreased, nausea and vomiting are less common.
- Nutritional deficiency:
- Women with hyperemesis gravidarum have been shown to be deficient in vitamin B6, and it has been considered whether this is a possible aetiology.
- Evolutionary adaptation:
- It has been suggested that nausea and vomiting are a mechanism to prevent the woman eating potentially harmful foods. The temporary aversions to tastes and smells that some pregnant women experience may support this theory.
- Gastric dysfunction:
- In pregnant women, oesophageal, gastric, and small-bowel motility are impaired because of smooth-muscle relaxation due to increased levels of progesterone. Delayed gastric emptying in pregnancy may also contribute to nausea and vomiting.
- Vestibular and olfactory system:
- It is postulated that a pregnancy stimulus may lower the threshold for vestibular-mediated nausea and vomiting in some women. Hyperacuity of the olfactory system is thought to be due to increasing oestrogen concentrations in early pregnancy.
- Psychological influences:
- Less favoured is the theory that nausea and vomiting in pregnancy are an abnormal response to stress or are associated with negative feelings about the pregnancy. It has been suggested that psychological symptoms may be a result, rather than a cause, of nausea and vomiting in pregnancy.
[Eliakim et al, 2000; ACOG, 2004; Davis, 2004; Festin, 2006]
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