Morning sickness

Causes and risk factors

The exact cause of morning sickness is unknown, but research suggests a number of different causes.

Increased oestrogen and progesterone levels

Changes in levels of the female sex hormones during early pregnancy may cause short-term nausea and vomiting. Oestrogen– at its highest level during the first 12 weeks when the symptoms of morning sickness are worst – can heighten your sense of smell, which explains why smells such as food cooking can make you feel sick. Progesterone production also increases, making the muscles in your small intestine, oesophagus and stomach relax – another cause of nausea.

HCG levels

After conception, the body begins to produce a hormone called human chorionic gonadotrophin. It is thought that higher levels of this hormone may cause morning sickness.

Nutritional deficiencies

A lack of vitamin B6 in the diet is thought to be another possible cause of morning sickness. You should be able to get enough through a healthy, balanced diet that contains food such as whole cereals, oatmeal, wheat germ and brown rice, wholemeal bread, cod, soya beans, milk, potatoes, peanuts, vegetables and bananas.

Although vitamin B6 appears to be effective at reducing the symptoms of morning sickness, there is limited evidence regarding its safety during pregnancy. Therefore, you shouldn't take over-the-counter remedies or supplements without talking to your midwife or GP first.

It is thought that a lack of vitamin B6 may cause hyperemesis gravidarum, a rare but severe form of nausea and vomiting in pregnancy.

Risk factors

A number of different factors increase the chances of having nausea and vomiting. These include:

  • your first pregnancy
  • having morning sickness in a previous pregnancy
  • having a family history of morning sickness
  • your unborn baby being a girl
  • having a multiple pregnancy, such as twins or triplets
  • having a history of motion or travel sickness
  • experiencing nausea while using contraceptives containing oestrogen
  • having a body mass index (BMI) of 30 or more
  • stress.