Antiphospholipid syndrome (APS). This is also known as Hughes' syndrome. It is caused by certain immune system chemicals (antibodies) in the blood, which are called antiphospholipid antibodies. APS can cause a blood clot to form in arteries and small blood vessels, as well as in veins. APS can affect pregnancy in some cases. Many women with APS do not have problems in pregnancy. However, APS may cause miscarriage, or other problems - growth restriction of the fetus, pre-eclampsia or, rarely, fetal death. These problems can be reduced by treatment.
APS can be treated with low-dose aspirin, which is helpful in pregnancy. If you have had a blood clot then warfarin is usually advised instead (or heparin if you are pregnant).
Other conditions can increase the risk of a blood clot. Some doctors classify these as thrombophilias. Examples are certain disorders affecting platelets, some bone marrow disorders, some kidney problems, inflammatory bowel disease and advanced cancer.