We do not yet know what causes the restriction of the flow of bile from the liver in OC. Evidence suggests that it is caused by a combination of hormonal, genetic and environmental factors2.
Hormonal factors
All hormones are metabolised (broken down) in the liver. One theory is that the liver cannot cope with the high levels of hormones during pregnancy (oestrogen and progesterone). This affects the liver’s ability to remove bile salts efficiently and causes a build up of bile salts in the blood. This is supported by observations that:
- OC is more common in twin and triplet pregnancies, which are associated with higher levels of hormones4.
- OC has been triggered in women taking high-dose oral contraceptives (which contain forms of oestrogen and progesterone) and also in women being treated with progesterone2.
Genetic factors
OC may also have a genetic cause. This means it may be linked to an abnormality or ‘mutation’ in a gene or genes. Genetic factors could explain cases where OC occurs within families and also why it is more common in some ethnic groups. Although OC affects one in 140 (0.7%) pregnancies in the UK overall, it is more common in women of Indian or Pakistani origin, affecting around one in 70 to 80 (1.2 – 1.5%)5 pregnancies. In other countries, such as South America and Scandinavia, the number of women affected is higher still6.
Some gene mutations have been identified in women with OC and it is likely that they make some women more susceptible to the disease7. However, this does not explain all the causes of the disorder and other factors such as diet and hormones may play a part. Further research is being carried out to investigate these areas.
Environmental factors
Some characteristics of OC suggest that environmental factors may also have a role.
- OC does not always recur in following pregnancies.
- OC can be more common in certain seasons (in the winter months in Chile and Scandinavia).
- Cases of OC have decreased in countries where nutrition has improved4,6.
Other factors
Women with hepatitis C more commonly develop OC than those who do not have the virus8.
What role do bile salts play in OC?
Although raised bile salts have previously been thought to be the cause of the itch associated with OC, research has so far not confirmed this link. A recent study suggests that the itch may be caused by another chemical in the blood (lysophosphatidic acid) which is raised in women with OC compared to pregnant women without complications9.
What is known is that the bile salts can cross the placenta and may be linked to the reason why some babies suffer complications10 or are stillborn. There is evidence from human and animal studies that bile salts cause structural damage to the placenta. Further research is required to better understand this8.
There have been no reports of any harmful effects to babies from OC pregnancies once they have been delivered.