How are twins formed?

Identical twins (or monozygotic twins)

·         Identical twins happen when a single sperm fertilises an egg, and then, at a very early stage, the fertilised egg divides into two and starts forming two babies. 

·         Identical twins have the same genes, so they are the same sex. 

·         Some of these twins have their own separate placenta (afterbirth) and sac to grow in the mother’s uterus (womb) but many share the same placenta and a few also share the same sac. 

·         Monozygotic twins may be quite different sizes at birth, but they become more alike with time. They are often hard to tell apart when they are older. 

·         Identical twins do not usually ‘run in families’ and it is not known why monozygotic twins occur. 

·         Identical twins happen about once in every 250 live births.

Non-identical twins (or dizygotic twins)

·         Non-identical twins are also sometimes called fraternal twins. They happen when two separate eggs are fertilised by two different sperm so that two embryos (the beginnings of a baby) are formed. 

·         Each has its own separate place in the uterus and separate placenta and sac. 

·         They may be the same sex or different sexes. 

·         Dizygotic twins are often similar at birth, but they become less alike as they get older, as do other (different age) brothers and sisters. 

·         Dizygotic twins are more likely to happen when there are twins in the mother’s family, or if the mother has been having fertility treatment. If a mother is a non-identical twin, she has about a 10% chance of having twins herself. (A mother of twins who is not a twin herself has about a 5% chance of having another set of twins). If the father is a twin, this does not make it more likely that the parents will have twins. 

·         About two births in every hundred are dizygotic twins. There has been a rise in the number since the use of fertility treatment has become more common.

Siamese twins (or conjoined twins)

·         Siamese twins are rare. 

·         They are twins who come from the same egg (monozygotic) but the embryo does not separate completely to form two separate babies. 

·         This means that the babies are joined together in some way. Some can be separated by an operation without too much difficulty. In other cases they share vital organs and cannot be separated without the death of one or both twins.

A vanishing twin
With ultrasound being done early in pregnancies, it has been found that many more pregnancies start as twin pregnancies (up to 5% at 12 weeks), but one of the babies stops developing. The other baby develops normally.  

Knowing if twins are monozygotic or dizygotic
Usually your doctor will work out whether your twins are monozygotic or dizygotic soon after the birth. If they are different sexes they are definitely dizygotic. Monozygotic twins are more likely to have some physical problems at birth, and also more likely to have similar health problems when they are older – so it can be useful to tell if they are identical or not. Since monozygotic twins can look very different at birth, the doctor may have the placenta very carefully examined, or do blood tests.