A team of scientists have identified a hormone, produced by the foetus, that triggers severe morning sickness characterised by nausea and vomiting in the first trimester of pregnancy.

The team from the University of Cambridge found GDF15 -- a hormone acting on the brain stem -- causing nausea and vomiting during pregnancy (NVP) in about seven in ten women.

However, about 2 per cent of women suffer the most severe form -- Hyperemesis Gravidarum (HG), leading to weight loss, dehydration and hospitalisation.

The new findings may help develop treatments for the pregnancy sickness, the underlying cause of which had, until now, been unclear.

The study, published in the journal Nature, found that the severity of the morning sickness in pregnant women depends on a combination of how much GDF15 is produced by the foetus and how much exposure the mother had to this hormone before becoming pregnant.

"The baby growing in the womb is producing a hormone at levels the mother is not used to. The more sensitive she is to this hormone, the sicker she will become," Stephen O’Rahilly, from Wellcome-MRC Institute of Metabolic Science at the University of Cambridge.

"Knowing this gives us a clue as to how we might prevent this from happening. It also makes us more confident that preventing GDF15 from accessing its highly specific receptor in the mother’s brain will ultimately form the basis for an effective and safe way of treating this disorder," O'Rahilly added.

The team found that a rare genetic variant that puts women at a much greater risk of HG was associated with lower levels of the hormone in the blood and tissues outside of pregnancy.

Similarly, women with the inherited blood disorder beta thalassemia, which causes them to have naturally very high levels of GDF15 before pregnancy, experience little or no nausea or vomiting.

An experimental model exposed to acute, high levels of GDF15 showed signs of loss of appetite, suggesting that they were experiencing nausea, but the model treated with a long-acting form of GDF15 did not show similar behaviour when exposed to acute levels of the hormone.

The researchers believe that building up a woman’s tolerance to the hormone before pregnancy could hold the key to preventing sickness.

"Hopefully, now that we understand the cause of hyperemesis gravidarum, we’re a step closer to developing effective treatments to stop other mothers going through what I and many other women have experienced," said co-author Dr Marlena Fejzo from the Department of Population and Public Health Sciences at the University of Southern California.