The presence of more than one fetus in the uterus and the problems that may arise
- Multiple pregnancy more common over the age of 30
- Multiple pregnancy sometimes runs in families
- Lifestyle is not a significant factor
Multiple pregnancy occurs when more than one fetus develops in the uterus. Although most multiple pregnancies progress smoothly, the risk of problems is increased for both mother and fetuses.
Nonidentical fetuses may develop if two or more eggs are fertilized at the same time, whereas two or more identical fetuses may develop if an egg splits after fertilization. Fetuses may share a placenta or each fetus may have its own.
In the UK, twins occur naturally in about 15 in 1,000 births and trip-lets occur in about 1 in 4,400 births. Natural pregnancies of more than three fetuses are extremely rare, but there has been a dramatic increase in multiple pregnancies as a result of fertility treatment.
You are more likely to have nonidentical twins if you are over the age of 30 or if there are multiple births in your family history on your mother’s side. However, these factors do not increase your chance of having identical twins.
What are the causes of multiple pregnancy?
Multiple pregnancies may occur naturally, but 2 in 3 pregnancies with three or more fetuses result from infertility treatments (see Assisted conception). This is because drugs for infertility stimulate the ovaries to release more than one egg each month, and more than one egg is inserted into the uterus during in-vitro fertilization.
What are the problems?
The common problems associated with normal pregnancy (see Common complaints of normal pregnancy) are sometimes more severe in a multiple pregnancy. This is partly due to the increased size of the uterus and partly because the placenta or placentas produce a higher level of hormones.
Women who are carrying more than one fetus are more likely to have certain problems, such as high blood pressure during pregnancy (see Pre-eclampsia and eclampsia). Such women are also more likely to develop excessive vomiting during early pregnancy (see Hyperemesis), have greater than normal amounts of fluid surrounding the fetus (see Polyhydramnios), and have a low-lying placenta or placentas (see Placenta praevia). Iron-deficiency anaemia is also more likely because the mother must supply iron to more than one fetus.
A multiple pregnancy is more likely to result in a miscarriage or in premature labour. The fetuses in a multiple pregnancy are often smaller than single babies, and there is a greater risk of one baby not being in the normal position for labour (see Abnormal presentation). Since there may be more than one placenta, bleeding after the delivery may be heavy (see Postpartum haemorrhage).
What might be done?
Your doctor or midwife may suspect a multiple pregnancy if your abdomen is larger than would be expected for the stage of the pregnancy. The diagnosis is usually confirmed by ultrasound scanning (see Ultrasound scanning in pregnancy). You may be monitored with ultrasound scans every month, possibly with Doppler ultrasound scanning every 2 weeks to check that each fetus is receiving adequate blood. Some hospitals have multiple pregnancy clinics.
Additional treatment may be needed if problems develop. For example, you may be prescribed iron supplements to treat anaemia. You should rest often, especially after the 28th week, to help to reduce the risk of a premature labour. Multiple fetuses are usually delivered in hospital because of the risks of a difficult or premature labour. Twins may be delivered vaginally if the first baby is in the head-down position. A caesarean section is usually necessary for the delivery of three or more babies.
Since the babies are often premature and small, they may need to be looked after in a special care baby unit.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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