High blood pressure during pregnancy is a world-wide health problem that can be dangerous to mothers, and commonly causes premature birth and small babies. There is also growing evidence that mothers who suffer from high blood pressure in pregnancy and their babies both have a higher risk of high blood pressure and cardiovascular disease in later life. Previous studies have revealed detrimental changes in the structure and function of the heart and blood vessels of mothers and their babies who experience this common complication which may be important in understanding the subsequent increased risk of later disease. The investigators have also learned through previous work carried out by our research group that certain biological processes may be disturbed in women with high blood pressure in pregnancy. One of these processes involves a molecule called tetrahydrobiopterin (also known as BH4) which is required for the correct functioning of blood vessels. Supplementation with biologically-active forms of reduced folate are able to increase the levels of this molecule and can reverse the changes caused by high blood pressure in pregnant mice, and in cultured human endothelial cells. These particular folate supplements are safe and approved for use in pregnancy, but are not widely used as they are more specialised and currently less available than folic acid, the oxidized form of folate that is widely used for supplementation in pregnancy. However, folic acid does not have the same effects as the specific biologically-active form of reduced folate (tetrahydrofolate), because folic acid requires enzymatic conversion within the body in order to generate the biologically-active form of reduced folate. Whilst the use of folic acid appears to be sufficient in most cases to increase folate levels in the prevention of fetal abnormalities such as neural tube defects, the conversion of folic acid to its biologically-active reduced form may be itself deficient or detrimental in women with preeclampsia, and does not increase tetrahydrobiopterin levels. Indeed, folic acid treatment has been found to be ineffective in a large clinical trial to improve high blood pressure in pregnancy women.

This clinical trial will investigate the levels of biologically-active forms of reduced folates. As a way to increase the levels of tetrahydrobiopterin, in pregnant women with high blood pressure, women participating in the trial will take either a low dose or a high dose of a biologically-active form of reduced folate (tetrahydrofolate supplement) until delivery of their baby. A further group of women with high blood pressure will be recruited and will take a placebo. Blood tests will be taken and measurements performed to evaluate the heart and blood vessels of the participating women and their babies upon entry into the study and again at the time of birth. The investigators will also collect the umbilical cord and placenta tissue samples once the baby has been born, so that the investigators can assess blood vessel cell function following tetrahydrofolate supplementation in laboratory studies. In addition to the groups of women with high blood pressure, the investigators will also invite a group of women with an uncomplicated pregnancy to take part, as a comparison group, these women will not receive any supplementation. The investigators aim to investigate how levels of factors in the folate pathway differ in pregnant women with high blood pressure and if this is reflected in functional changes in the heart and blood vessels of these women. The investigators also aim to study whether this tetrahydrofolate supplement can increase the levels of tetrahydrobiopterin and if this improves blood vessel function. If successful, the investigators hope that the results of this study will enable us to design and conduct a larger study in future in order to test whether taking this tetrahydrofolate supplement will stop or reverse the changes caused by high blood pressure in pregnancy.