Miscarriage

A miscarriage is the loss of a pregnancy before a fetus becomes viable. Pregnancy losses that occur later on, from about 20 weeks’ gestation onwards, are called stillbirths.

Until recently, about 25 per cent of pregnancies were thought to end in miscarriage. But now, scientists are starting to realise the rate is actually much higher. The latest estimate is that 60 per cent of pregnancies actually end in miscarriage. In developed countries, the average woman has an estimated 2.5 miscarriages in her lifetime.

Around three-quarters of these losses occur before pregnancy symptoms like missed periods and nausea emerge, so they often go unnoticed. Early miscarriages typically present like a normal period, or one that is slightly heavier or later than usual. We are more aware of them than in the past because of the availability of “pee-on-a-stick” tests that can detect pregnancies from up to 6 days before a missed period.

Later miscarriages are more obvious, because blood clots and pregnancy tissue may also be discharged and cramping may be more severe.

Pregnancy losses, regardless of when they occur, are often distressing. One UK study found that 28 per cent of women and 12 per cent of men felt anxious after having a miscarriage and 10 per cent of women and 4 per cent of men felt depressed. The especially high rates of anxiety are thought to relate to uncertainties about how hard it will be to get pregnant again and whether it will stick next time.

Fortunately, most women who have miscarriages go on to have successful pregnancies. Only 1 to 2 per cent of women experience recurrent miscarriage, meaning the loss of three or more pregnancies in a row, and three-quarters of these women eventually have successful pregnancies.

It’s common for women who have had miscarriages to worry it was their fault. One US survey found that 41 per cent of women who lost a pregnancy thought they had done something wrong. However, there is no evidence that exercise, prior birth control use, working or having sex cause miscarriages.

Miscarriages are sometimes triggered by infections with viruses like the flu, malaria, dengue or HIV. They are also more common in women with conditions like endometriosis and thyroid disease or physical problems with their uterus.

However, most miscarriages are caused by chromosomal errors that occur when a sperm and egg come together to form an embryo. Because chromosomes contain all the genes needed to build a functioning human being, embryos with such errors cannot develop normally. Scientists still don’t know why chromosomal errors are so common in human embryos.

Chromosomal abnormalities in embryos increase as women get older, which is why maternal age is thought to increase miscarriage risk. The miscarriage rate for women who conceive in their early 20s is around 50 per cent, compared to 80 per cent for those who conceive in their early 40s.

It’s not clear how women’s bodies decide which embryos to keep and which ones to lose. But increasing evidence suggests that the endometrium – the lining of the uterus – acts like a quality sensor. If the endometrium detects a healthy embryo, it encourages it to implant and grow, whereas if it detects an abnormal embryo, it stops it from sticking properly. Understanding how this quality sensor works and how it sometimes goes wrong may lead to new treatments for women with recurrent miscarriage or infertility.