3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> How twins are formed

How twins are formed

The discovery that you are expecting twins can be a shock. It is perfectly natural to worry about how you will cope with two babies, but you will also have a real sense of excitement. There is something different about expecting twins: other mothers to be will want to ask you what it is like, and you will receive extra care from medical professionals as well, adding to the feeling that something special is happening.

The likelihood of twins

If you have undergone fertility treatment, you will have a greater chance of conceiving more than one baby. Fertility drugs such as clomiphene make you more likely to release more than one egg at a time, while in the case of in vitro fertilization (IVF,) more than one embryo is usually implanted, so you may end up having twins or triplets. On average, 25 per cent of women using either fertility drugs or IVF conceive twins.

You are also more likely to conceive fraternal twins if there is a family tendency to do so, especially through the maternal side. Your age is also a factor in this: women who over 30 years old produce more follicle-stimulating hormones than earlier in life so they are more likely to release more than one egg at a time when they ovulate.

Finding out if you have twins

If you have had no reason to suspect that you are expecting twins, you may not have a routine ultrasound scan until you are 12 weeks or more pregnant, by which time the two babies will be clearly distinguishable in the image. If you have had fertility treatment, you will probably have an early ultrasound scan at around 6 weeks into your pregnancy, and even at this stage of your pregnancy you will be able to see two tiny heartbeats on the monitor screen.

Identical or not?

There are two types of twins: identical and fraternal (non-identical). Which type a woman has depends on how they were conceived.

Identical twins occur when one egg splits into two after it has been fertilized by a sperm. Because the twins develop from two halves of the same fertilized egg, they contain identical genetic information, so they will be the same sex, look very alike and even have similar personalities. Around one in three sets of twins are identical, and two-thirds of these will share a placenta while in the uterus. Each has their own amniotic sac.

Fraternal (non-identical) twins occur when a woman produces two separate eggs that are then fertilized by two different sperm.These babies will not be any more or less alike than any other pair of siblings, and each will be contained within their own amniotic sac and have their own placenta, although the latter may fuse during the pregnancy. Very rarely, a single egg may split before being fertilized by two different sperm. In this case, the babies are likely to be very similar although not identical because only half of their genetic information will be the same.

Conjoined twins

If a single egg fails to split properly after fertilization, conjoined twins (sometimes known as Siamese twins) will develop. Conjoined twins occur only very rarely -about once per 200,000 live births. There are various types, depending on where the twins are joined. Although it may be possible to separate conjoined twins after birth, the condition carries serious health risks, especially if the twins are sharing vital organs or have a complex shared blood supply.


Triplets are becoming more common as a result of fertility treatments. However, in some countries, women under 40 can only have two embryos implanted with each IVF cycle, so the most they will conceive is twins. There are three types of triplets:

Three identical triplets if one egg splits into three after fertilization. Two identical and one non-identical triplets if two eggs are released and only one splits in two. Three non-identical triplets if three separate eggs are fertilized.