| Twins can be the same (identical-monozygotic) or | | | | increases. To put it in perspective, your chance of |
| different (fraternal-dizygotic). Fraternal twins, triplets, or | | | | giving birth to twins if you are less than 25 years of |
| more develop when two or more eggs are fertilized | | | | age is less than half of what it would be after the age |
| by separate sperm. Fraternal fetuses have separate | | | | of 35. |
| placentas and amniotic sacs (the bag containing the | | | | 2. You are more likely to conceive twins in the first |
| fluid around a baby). | | | | few menstrual cycles after stopping birth control pills. |
| Identical twins come from a single egg that has been | | | | 3. Infertility treatment. Fertility drugs stimulate your |
| fertilized by one sperm. For unknown reasons, the | | | | ovaries to release multiple eggs, which can be fertilized |
| fertilized egg splits into two or more embryos during | | | | at once. This may result in a multiple pregnancy. You |
| the first stage of development. Some identical twins | | | | are, therefore, more likely to have a multiple pregnancy |
| share the same placenta. However, they usually grow | | | | if you use fertility drugs or assisted reproductive |
| within separate amniotic sacs in the uterus. In rare | | | | technology (ART - In Vitro Fertilization IVF). |
| cases, identical twins share one amniotic sac | | | | 4. ART is the transfer of embryos directly into the |
| (monoamniotic). | | | | uterus. In the early days, when success rates were |
| Fraternal multiples: | | | | extremely low, several embryos were transferred in |
| 1. Can be of different sexes and may have different | | | | the hope of achieving a pregnancy. Nowadays, 25% |
| blood types. | | | | to 30% of pregnancies from ART are twins; 5% are |
| 2. They may look very different from one another, | | | | triplets and less than 1% are quadruplets or more. As |
| with different-coloured hair and eyes. They may also | | | | techniques have improved, success rates from IVF |
| look alike, as siblings often do. | | | | embryo transfer have increased and reasonable |
| 3. Tend to run in families. | | | | success rates are being achieved with single embryo |
| 4. They are more common in older mothers, African | | | | transfer and this reduces the incidence of multiple |
| races and when there is a family history of twins. | | | | pregnancies. |
| They are least common in Asian families. If you have | | | | What is the prevalence of multiple pregnancy? |
| already carried fraternal twins, you have double the | | | | The natural incidence of twins is 1 in 80 pregnancies |
| usual risk of having twins. A history on your partner's | | | | and for triplets 1 in 8000 pregnancies. The incidence is |
| side of the family does not increase your risk of | | | | rising as a result of IVF and a tendency for women to |
| multiple pregnancy. | | | | deliver later in their reproductive years. Naturally |
| Identical twins | | | | occurring quadruplet births occur in 1 per 600,000 births. |
| 1. Are always the same sex and blood type. | | | | The UK would, therefore, expect one set of naturally |
| 2. They do not necessarily look exactly alike. One may | | | | occurring quadruplets each year. There are estimated |
| be right-handed while the other is left-handed. | | | | to be approximately 125 million human twins and triplets |
| 3. They develop at random. They are not related to | | | | in the world, and just 10 million identical twins. |
| maternal age, race, or family history of twins. | | | | One in two pregnancies with same sex babies are |
| 4. Multiples that are conceived with infertility treatment | | | | monozygous (identical twins). The rate of identical |
| are usually fraternal, rather than identical. | | | | twins remains at about 1 in 333 pregnancies across |
| What are the causes of multiple pregnancy? | | | | the globe. |
| 1. With increasing maternal age the chance of twins | | | | |