Twins can be the same (identical-monozygotic) or different (fraternal-dizygotic). Fraternal twins, triplets, or more develop when two or more eggs are fertilized by separate sperm. Fraternal fetuses have separate placentas and amniotic sacs (the bag containing the fluid around a baby).

Identical twins come from a single egg that has been fertilized by one sperm. For unknown reasons, the fertilized egg splits into two or more embryos during the first stage of development. Some identical twins share the same placenta. However, they usually grow within separate amniotic sacs in the uterus. In rare cases, identical twins share one amniotic sac (monoamniotic).

Fraternal multiples:

1. Can be of different sexes and may have different blood types.

2. They may look very different from one another, with different-coloured hair and eyes. They may also look alike, as siblings often do.

3. Tend to run in families.

4. They are more common in older mothers, African races and when there is a family history of twins. They are least common in Asian families. If you have already carried fraternal twins, you have double the usual risk of having twins. A history on your partner's side of the family does not increase your risk of multiple pregnancy.

Identical twins

1. Are always the same sex and blood type.

2. They do not necessarily look exactly alike. One may be right-handed while the other is left-handed.

3. They develop at random. They are not related to maternal age, race, or family history of twins.

4. Multiples that are conceived with infertility treatment are usually fraternal, rather than identical.

What are the causes of multiple pregnancy?

1. With increasing maternal age the chance of twins increases. To put it in perspective, your chance of giving birth to twins if you are less than 25 years of age is less than half of what it would be after the age of 35.

2. You are more likely to conceive twins in the first few menstrual cycles after stopping birth control pills.

3. Infertility treatment. Fertility drugs stimulate your ovaries to release multiple eggs, which can be fertilized at once. This may result in a multiple pregnancy. You are, therefore, more likely to have a multiple pregnancy if you use fertility drugs or assisted reproductive technology (ART - In Vitro Fertilization IVF).

4. ART is the transfer of embryos directly into the uterus. In the early days, when success rates were extremely low, several embryos were transferred in the hope of achieving a pregnancy. Nowadays, 25% to 30% of pregnancies from ART are twins; 5% are triplets and less than 1% are quadruplets or more. As techniques have improved, success rates from IVF/embryo transfer have increased and reasonable success rates are being achieved with single embryo transfer and this reduces the incidence of multiple pregnancies.

What is the prevalence of multiple pregnancy?

The natural incidence of twins is 1 in 80 pregnancies and for triplets 1 in 8000 pregnancies. The incidence is rising as a result of IVF and a tendency for women to deliver later in their reproductive years. Naturally occurring quadruplet births occur in 1 per 600,000 births. The UK would, therefore, expect one set of naturally occurring quadruplets each year. There are estimated to be approximately 125 million human twins and triplets in the world, and just 10 million identical twins.

One in two pregnancies with same sex babies are monozygous (identical twins). The rate of identical twins remains at about 1 in 333 pregnancies across the globe.