Why do miscarriages happen?

A miscarriage is when a pregnancy is lost before 24 weeks gestation. Early miscarriage refers to pregnancy loss within the first 12 weeks, and it is much more common than late miscarriages.

The chance of having a miscarriage in the first three months is about 20%, and this often occurs by chance, not impairing future chance of successful pregnancy.

Much is still not known why early miscarriages occur. Studies have shown that majority (as high as 70%) of cases are due to underlying chromosomal / genetic defects. In about 1 in 100 women will experience recurrent miscarriages (defined as three or more consecutive miscarriages). There are a number of conditions that may play a part in recurrent miscarriages. These include

Age and past pregnancies – increased maternal age is associated with greater risk of miscarriage, and the more miscarriages one had before, the higher the chance of suffering from one again.

Genetic factors – about 3% of women who have recurrent miscarriages will have a chromosomal abnormality found in them or their partner. These abnormalities are often not associated with health problems to the affected person. However, when passed on to the baby, it may result in a miscarriage.

Autoimmune factors – about 15% of women who have recurrent miscarriages have abnormal antibodies called antiphospholipid antibodies in their blood. These antibodies react to the body's own tissues and cause an autoimmune response.

Womb structure – problems and abnormalities with the womb can lead to both recurrent miscarriages and late miscarriages. Possible problems include non-cancerous growths called fibroids (sited inside the womb cavity), or congenital anatomical abnormalities. Minor variations in the structure of the womb do not cause miscarriages.

Weakened cervix – in some women, the opening of the womb (cervix) opens too early in the pregnancy and causes a miscarriage in the second trimester. This is known as cervical incompetence.

Polycystic ovaries – Polycystic ovary syndrome (PCOS) is a condition where the ovaries are larger than normal and produce more immature follicles than normal. This may be associated with hormonal imbalance inside the womb.  However the exact role that PCOS plays in miscarriages is unclear.

Hyperprolactinaemia – prolactin is a hormone that is produced during pregnancy. The hormone prepares the breasts to produce milk. Some women overproduce this hormone and this could be linked to an increased risk of miscarriage.

Infections – there are some infections that may increase the risk of miscarriage, and these include vaginal infections like bacterial vaginosis, as well as systemic infections like cytomegalovirus, toxoplasmosis and sexually transmitted infections.

Blood conditions – certain inherited blood disorders (thrombophilia) may cause blood to clot more easily than usual. How these conditions cause miscarriages remains unknown.

Medical problems – diabetes or thyroid problems can be factors in single miscarriages. They do not usually cause recurrent miscarriage if well controlled.

Misconceptions about miscarriages

There is no evidence to support claims that a miscarriage is linked to exercise, sex, lifting or straining during pregnancy and mother’s emotional state during pregnancy.

Publication of this article by courtesy of Dr Ben Choey, Obstetrics and Gynaecology Specialist

From SBCC Women's Clinic at Clementi.