Pregnant with Twins

In recent years, fertility treatments and the fact that women are having babies later has made multiple births more common.

Different types of twins

One-third of all twins will be identical and two-thirds non-identical.

Identical twins

Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in two, creating identical twins who share the same genes. Identical twins are always the same sex, so if your twins are identical, you'll have two girls or two boys and they'll look very alike.

Non-identical twins

Non-identical (dizygotic) twins happen when two separate eggs are fertilised and then implant into the woman's womb (uterus).  These non-identical twins are no more alike than any other two siblings.  Non-identical twins are more common.  The babies may be of the same sex or different sexes.

Are you carrying twins?

You might suspect that you are carrying more than one baby if:

  • you seem bigger than you should be for your dates
  • twins run in your family
  • you have had fertility treatment

It is usually possible to find out if you're having twins through your dating ultrasound scan, which happens when you are 8-14 weeks pregnant.

At the scan, you should be told whether the babies share a placenta (meaning they are identical) or if they have two separate placentas (meaning they can be identical or not).  If this is not clear from the first scan, you should be offered another one.

One-third of identical twins have separate placentas.  This occurs when the fertilised egg splits before implanting in the uterus, up to four days after conception.

What causes twins?

Nobody knows what causes identical (monozygotic) twins.  All pregnant women have approximately the same chance of having identical twins – about 1 in 350-400.  Identical twins do not run in families.

However, there are some factors that make having non-identical twins more likely:

  • non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese
  • older mothers are more likely to have non-identical twins because they are more likely to release more than one egg during ovulation
  • non-identical twins run on the mother's side of the family, probably because of an inherited tendency to release more than one egg

In vitro fertilisation (IVF) can increase the chance of twins as more than one embryo may be transferred .

How can I tell if my twins are identical?

The most accurate way to tell if twins are identical is through a DNA test.  This can only be done after your babies are born.

The placenta could also provide clues.  If your first ultrasound scan is done before 14 weeks, it should be possible to tell accurately what kind of placenta your twins have. Otherwise, the placenta can be examined after your babies are born.  However, neither of these methods is foolproof.