Clinical guidelines currently recommend that pregnant women should avoid statins, but the advice is based on the assumption that cholesterol is essential for normal fetal development and clinicians do not know that using statins during pregnancy could do more harm than simply lowering cholesterol.

A 2007 study has already suggested that fat soluble or lipophilic statins may increase the risk of congenital anomalies in children of pregnant women.   But it is unknown whether water-soluble statins would have the same detrimental effects.

A new study led by researchers from the University of Manchester has now showed that water-soluble or hydrophilic statins like prevastatin can also affect placental development potentially resulting in worse pregnancy outcomes.

Obesity and type 2 diabetes patients are often given statins to lower circulating levels of cholesterol and reduce the risk of heart disease. The study suggests that pregnant women should not be given any type of statin.

Dr Melissa Westwood at the Maternal and Fetal Health Research Centre at St Mary's Hospital, Manchester said the actions of statins are not limited to the regulation of cholesterol levels and they can affect the production of other chemicals in the body as well.

"Our study examined the effects that both lipophilic and hydrophilic statins had on a key biological system that is crucial for maintaining the normal function of the placenta, which acts as the nutrient-waste exchange barrier between mother and fetus."

Dr. Westwood and colleagues tested two statins, one water soluble and the other fat soluble in a placental-tissue model.   And they found the fat -soluble statin, cerivastatin reduced growth of the placenta, which was expected.

Surprisingly, they also found that prevastatin, the water soluble stain which had been believed to be suitable for use in pregnancy, had the same detrimental effect.

"These results clearly show that the effect of statins on the placenta is not dependent on their lipophilicity as had previously been suggested," said Dr Westwood.

"While hydrophilic statins have not been reported to increase the incidence of fetal malformations, our research suggests that they will have a detrimental effect on placental growth, which is likely to result in poor pregnancy outcome.

The findings justify the recommendation that pregnant women should avoid the use of any type of statin once they plan to start a family or when a pregnancy is suspected or confirmed.