Twins happen when more than one baby develops in the womb. There are 2 types of twins: non-identical (when more than one egg is fertilized) and identical (when one single fertilized egg splits in early pregnancy).

Twins have become more common over the past 20 years – largely due to the use of fertility treatments.

There is no way you can choose to have twins, but some factors can make it more likely to happen:

Assisted reproduction technology (ART) – such as in vitro fertilization (IVF), where eggs are fertilized outside the body. A number of embryos might be produced and the couple may have the opportunity to have one or more transferred back into the womb. If they choose two or more to be transferred, there is a chance that more than one of these embryos will implant and the couple will have more than one baby

Medicines – women who take ovulation inducing drugs (e.g. clomiphene citrate) which stimulate the ovaries to produce eggs, have a higher chance of having twins or a multiple birth

Age – women over the age of 35 have a higher chance of having twins. This is because older women produce more follicle stimulating hormone (FSH) as they age, which can cause more than one egg to be released during ovulation

Heredity – if a woman is a twin, or has twins in her family, then she has an increased chance of having twins herself. This is due to an inherited tendency to release more than one egg during ovulation.