Morning sickness 'is inherited'

The “curse of morning sickness” runs in the family, according to the Daily Mail. The newspaper said that “women whose mothers suffered severe morning sickness in pregnancy are three times more likely to go through the same ordeal.”

The research behind this story analysed data from Norway’s national birth registry to investigate whether the risk of having severe morning sickness (medically known as hyperemesis gravidarum) was passed from mother to daughter. Scientists found that women had an increased risk of the problem if their mother had experienced severe morning sickness during any of her pregnancies. A history of morning sickness in the male parent’s mother made no difference to his female partner’s chances of having it, suggesting that the genetics of the foetus do not contribute to the risk. Ultimately, this study may lead to further research that can clarify whether a genetic or common environmental link explains the pattern seen.

Although milder feelings of nausea and vomiting occur in half of all pregnancies, women should consult their GP or midwife if their sickness is more severe. Medical staff can then ensure that expectant mothers maintain appropriate levels of nutrition during their pregnancy.

Where did the story come from?

Dr Ase Vikanes and colleagues from the Norwegian Institute of Public Health carried out this research. The study was funded by the Norwegian Research Council and published in the peer-reviewed British Medical Journal.

The research was generally covered accurately by the newspapers. However, the Daily Mail placed undue emphasis on this type of morning sickness being the result of environmental factors. The study did not provide sufficient evidence to say whether the risk of severe morning sickness is genetically passed from mother to daughter or due to a shared environmental factor.

What kind of research was this?

At least half of all pregnant women experience some degree of nausea and vomiting. The medical name for morning sickness, hyperemesis gravidarum, is applied to a more severe type of morning sickness which occurs in less than 2% of pregnancies. This severe morning sickness, which was the subject of this study, can lead to vitamin and nutritional abnormalities, maternal weight loss and pregnancy complications if it is not treated.

This cohort study looked at whether women whose mothers had experienced this type of severe morning sickness were more likely to have similar morning sickness during their pregnancy.

The study also looked at whether a mother’s risk of severe morning sickness was linked to her partner’s mother having the condition during pregnancy. A theoretical cause for such a link might be that the genetic make-up of the foetus (part of which would come from its father) would influence the pregnant mother’s chance of having morning sickness.

What did the research involve?

The researchers used the Norwegian medical birth registry, which has kept details of all births since 1967, to study the occurrence of birth outcomes across generations. An antenatal card is completed for all pregnant women at their first routine examination during the first trimester (first 12 weeks) of pregnancy. A midwife or doctor also completes a standardised form containing demographic data on the parents, maternal health before and during pregnancy, complications and interventions during delivery and the condition of the newborn baby. In Norway, each individual also has a unique national identification number.

The study had access to data on the 2.3 million births that occurred from 1967 to 2006. Using the national identification numbers, the study linked mothers to their childbearing daughters or child-producing sons. Hyperemesis was recorded in the birth registry and classified by international disease classification codes and criteria.

They assessed three models:

• Model 1 (mother-daughter recurrence) was a woman’s risk of hyperemesis if she herself was born after a pregnancy complicated by hyperemesis.
• Model 2 (mother-son recurrence) was the risk of hyperemesis in female partners of sons who were born after a pregnancy complicated by hyperemesis.
• Model 3 (mother-daughters recurrence) was the risk of hyperemesis in a woman born after a pregnancy not complicated by hyperemesis, but whose mother had hyperemesis in a previous or subsequent pregnancy.

What were the basic results?

The researchers found that if a mother had hyperemesis, the risk of hyperemesis in the daughter born to that pregnancy was 3%. The risk of a daughter having hyperemesis was 1.05% if her mother did not have hyperemesis. After adjusting for potential confounding factors of maternal age at birth, time period of birth in both generations and the number of previous children the mother had given birth to, they found that the risk of hyperemesis was 2.91 times greater for daughters whose mothers had experienced hyperemesis while carrying them (odds ratio [OR] 2.91, 95% confidence interval [CI] 2.36 to 3.59).

A woman’s risk of hyperemesis was not greater if her partner’s mother experienced hyperemesis while pregnant with him.

The researchers found that women also had an increased risk of hyperemesis if their mother had not had hyperemesis while carrying them but did have this severe morning sickness while carrying an older or younger sibling. The odds ratio for the daughter having hyperemesis if her mother had hyperemesis while carrying an older sibling was 3.18 (95% CI 1.56 to 6.49) and with a younger sibling 3.81 (95% CI 1.55 to 9.36).

How did the researchers interpret the results?

The researchers concluded that, “the risk of hyperemesis in a pregnant woman is threefold if the woman’s mother had ever experienced hyperemesis in a pregnancy. This was regardless of whether hyperemesis had occurred in the pregnancy leading to the woman under study or in a previous or subsequent pregnancy. In contrast, female partners of men whose mother had hyperemesis during pregnancy did not have an increased risk of hyperemesis”.

Conclusion

This study demonstrated a small increase in the risk of severe morning sickness (enough to cause weight loss and nutritional deficiencies) in the daughters of women who had severe morning sickness. One of this study’s strengths was that it included data from a whole population. Therefore, it did not feature any bias that may have arisen through selectively choosing only certain groups for inclusion.

However, the researchers highlight that their study did not have information on variables such as body mass index, smoking and ethnic background, which may have affected the outcomes. The Norwegian population may also have a different lifestyle and mix of ethnicities than the British population. Therefore, this study may not be directly applicable to British women.

As the researchers suggest, an increased risk across generations suggests that genetic factors may be important as the risk appears to be passed on to daughters. However, as the risk is not passed on to the female partners of sons, it is more likely that a pregnant mother’s genes have a greater effect on hyperemesis than the genetic make-up of their developing foetus. Importantly, as the researchers also said, it is possible that the risk “is not genetically transmitted but is caused by common environmental factors that are shared by mothers and daughters”. These might be nutritional or other lifestyle factors, or infections.

As this study showed that an increased risk was passed down from mother to daughter, further research is warranted into whether a woman’s genetic background can affect her likelihood of experiencing hyperemesis and the biological mechanisms underlying this condition.