Homocysteine results from the transmethylation of methionine. Its metabolism depends primarily on three enzymes and several vitamin cofactors. Genetic abnormality in these enzymes or deficiency of these vitamins lead to hyperhomocysteinemia (HHCh). HHCh is usually biologically defined by a fasting value >15 micromol/l. HHCh belongs among the congenital hypercoagulable states and is a long-known vascular disease risk factor. The discovery that HHCh may also be responsible for several pregnancy complications has only recently been made. Studies in this area are still scarce and report on limited numbers of patients. It nevertheless appears clear that HHCh is associated with the syndromes of repeated miscarriage, pre-eclampsia, placenta abruptio, thromboembolic events, neural tube defects, and perhaps with fetal death-in-utero and intra-uterine growth retardation. Supplementation with vitamin B9 can reduce plasma HC levels, and is thus recommended in patients with HHCh. The prevention of thromboembolic events during pregnancy by anticoagulant treatment is also desirable in these patients.