Blood clotting disorders

Although it has been known for a considerable time that a woman's blood becomes thicker in pregnancy, it has only recently been established that this process is more pronounced in some women compared with others. Blood clotting disorders (thrombophilias) can be inherited (genetic) or acquired.

Antiphospholipid antibodies, the two most important of which are the lupus anticoagulant and the anticardiolipin antibodies, cause blood to clot more easily. Women with a history of recurrent pregnancy loss who have persistently positive tests for either lupus anticoagulant and/or anticardiolipin antibodies are said to have Antiphospholipid Syndrome.

Inherited thrombophilias include Factor V Leiden, prothrombin gene mutations in the gene that codes for MTHFR, an enzyme involved in folate metabolism.

If blood clots occur in the blood vessels of the placenta, the blood flow to the baby is decreased and this can lead to either second trimester miscarriage or, if the pregnancy proceeds, to the birth of a baby that is smaller than he or she ought to be. Women with these disorders are also at risk of developing high blood pressure later in pregnancy.

For a long time, it was believed that thicker, "sticky" blood can cause first trimester miscarriages because it leads to blood clots in the developing placenta, preventing the embryo from getting the oxygen it needs. We now know that the first trimester placenta doesn't actually have any blood flowing through it- blood vessels are developing but they are plugged by placenta cells. In addition, our experience with embryos in IVF has taught us that oxygen is toxic to early embryos, so direct blood flow would be damaging.

It now appears that the antiphospholipid antibodies in acquired thrombophilias, or the proteins made by genes in inherited thrombophilias, prevent the placental cells from properly attaching to the mother's uterus. Without normal placenta development, the embryo cannot grow.