What causes recurrent miscarriage?

Miscarriage or abortion is the loss of pregnancy before the age of viability (24w gestation by WHO & 28w in sub-Saharan Africa). It is described as recurrent or habitual if there has been 3 or more first trimester losses or 2 or more second trimester losses. Recurrent miscarriage is known to affect 1% of couples attempting conception.
In 50% of cases, the cause of miscarriage is unknown. Other likely causes are chromosomal disorders such as translocation and aneuploidy; endocrine disorders such as polycystic ovarian disease, diabetes mellitus, hypothyroidism and luteal phase insufficiency; thrombophilia eg factor V Leiden and prothrombin mutation; immunological factor such as antiphospholipid syndrome, thyroid antibodies, increased uterine NK cells, male specific-minor histocompartibility; uterine malformations such as septum in the uterus; cervical factor such as incompetent cervix; ovarian factors eg reduced ovarian reserve & luteal phase defect; lifestyle factor such as smoking, alcohol & drugs and infections eg TORCH (toxoplasmosis, rubella, cytomegalovirus, herpes), coxsachie, measles and listeriosis.
Transvaginal scan has become the primary method of assessing the health of an early pregnancy. Parental chromosomal testing is advised in non pregnant patients who has had a recurrent miscarriage. Blood test should be performed for ovarian function, thyroid function, thrombophilia and diabetes mellitus.
Treatment is directed to identifiable cause with 60-70% success in subsequent pregnancy without treatment in cases of unexplained recurrent pregnancies. Close surveillance during pregnancy is generally recommended to pregnant patients with history of recurrent pregnancy loss.
Recurrent pregnancy loss is said to be associated with late development of coronary artery disease and women with recurrent pregnancy losses are at risk of developing pre-eclampsia (hypertension and proteinuria in 2nd half of pregnancy|) in later pregnancies.