Why do twins occur?

Twins can occur in two ways. In around a third of cases just one egg is fertilised but within days it splits, each half developing into an identical baby. In the remaining two-thirds, two eggs are released at the same time and both fertilised by separate sperm resulting in two non-identical babies.

Non-identical twins (also called fraternal, dizygotic, DZ or binovular) are no more alike than any other brothers or sisters. These twins are conceived at the same time or one following the other in a single menstrual cycle - so non-identical twins can even have different fathers. Non-identical twins share, on average, just half their genes, so they can look very like each other or totally different - most obvious when mixed race parents have white and black twins.

There is no single reason why this type of twinning occurs, but there are some associated factors:

• Racial origin (twins are more common among Nigerians, less common among Japanese)
• A family history on the mother’s side (the father’s contribution is less certain)
• Age - women in their late 30s are statistically more likely to have twins
• Whether you already have children - the likelihood of twins increases with the number of children already conceived

Identical twins (also called monozygotic, MZ or uniovular) occur when for no known reason the fertilised egg (zygote) splits during the first 14 days and each twin gets the same genetic make-up. This is usually simply a one-off event although recently it’s been noticed that slightly more identical twins have been born after the mother has had ovulation-inducing drugs - the treatment may make the egg’s outer layer more likely to split.

Conjoined (Siamese) twins are always identical and occur in extremely rare cases when the zygote divides after around 12-15 days.

Will they be identical?

You may have to wait until after the birth or even until your twins become toddlers to discover whether they are identical. The simplest way to tell if twins are non-identical is the most obvious - boy-girl pairs are never identical and a clear scan in mid-pregnancy will show you what you want to know. If you discover you are having twins at your first scan and it is before 12 weeks (it gets unreliable after this), ask the sonographer to check the placentas. A single placenta means the babies are most likely to be identical. Two placentas are inconclusive - they could be identical or fraternal.

After the babies are born the placenta may give more clues. If the placenta is one single unit the babies are identical, but two fused or two separate placentas mean they could be identical or fraternal. Blood can also be taken at birth from the babies’ umbilical cords and analysed for genetic markers such as blood group, serum protein or enzymes or, most accurate of all, a DNA test. However this isn’t available everywhere and you may well have to pay for it.

Finally, as your same-sex twins grow up, matching features like hair colour and texture, eye colour, ear shape and the timing of when their teeth come through will be fairly conclusive.