Chances of Having Twins

Two questions wearing one coat

"What are the chances of having twins" is really two questions, and they have different answers. Identical twins happen when one fertilised egg splits, which appears to be random. Non-identical twins happen when two eggs are released and both are fertilised, and that is influenced by several things you can name. Mixing the two is why the numbers you see quoted online rarely agree with each other.

The broad backdrop: RCOG says multiple pregnancy happens in about one in 60 pregnancies, and Tommy's puts it as around 1 in every 60 births in the UK being twins, triplets or more. The NHS adds that one-third of all twins are identical and two-thirds are non-identical.

Identical twins: the same chance for everyone

The NHS is unusually blunt about this. Nobody knows what causes identical (monozygotic) twins, everyone has about the same chance of having them, and that chance is about 1 in 250. The NHS also states plainly that identical twins do not run in families. Tommy's says the same thing and explains why: the split that creates identical twins seems to be random, cannot be predicted, and does not appear to be passed down.

So if your grandmother had identical twins, that is a nice family story. It is not a prediction.

Non-identical twins: what genuinely shifts the odds

Here the picture changes. The NHS lists factors that make non-identical (dizygotic) twins more likely, and each one comes back to the same mechanism: releasing more than one egg in a cycle.

Your age

The NHS says that if you are pregnant and over 35 you are more likely to have non-identical twins, because you are more likely to release more than one egg during ovulation. RCOG puts it more generally: multiple pregnancy is more common as you get older. Tommy's makes the same point about non-identical twins becoming more common with age.

Your family, on your mother's side

The NHS says non-identical twins run on the mother's side of the family, probably because of an inherited tendency to release more than one egg. Tommy's agrees that this can run in families. Note the direction of travel: it is the person carrying the pregnancy whose family history is relevant, because it is her ovulation that is affected.

Where your family is from

The NHS records that non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese. It gives no figures for individual groups, and neither will we.

Fertility treatment

The NHS says IVF can increase the chance of twins, as more than one embryo may be transferred. The HFEA is stronger still: it describes multiple birth as the main risk of fertility treatment, and has set clinics a target that less than 10% of their IVF births should be multiples. Many clinics now use elective single embryo transfer for exactly this reason.

Does it skip a generation?

This is the most repeated claim about twins and the least supported by any source we could verify. What the NHS and Tommy's actually say is narrower: identical twins do not run in families at all, and non-identical twins can run in families through the mother's line. Neither describes a generation-skipping pattern. If your family has non-identical twins in it, the inherited tendency is simply there or not.

Could you already be carrying twins without knowing?

The NHS lists three things that might make you suspect it: seeming bigger than you should be for your dates, twins running in your family, and having had fertility treatment. None of those confirms anything. The NHS says it is usually possible to find out through the 12-week scan, and that is genuinely the moment most people find out.

What about the things people swear by?

Diet, supplements, timing and positions all get credited with producing twins. None of the sources here supports any of it. The NHS lists exactly four things that make non-identical twins more likely, and they are age, family history on the mother's side, ethnic background and fertility treatment. Tommy's and RCOG add nothing to that list. Anything else you read is speculation attached to a mechanism nobody has demonstrated.

One genuine caveat: stopping hormonal contraception, breastfeeding and other things that influence ovulation are sometimes claimed to matter. The verified sources do not mention them in this context, which is not proof they do nothing, but does mean nobody should be selling you advice based on them.

If you have already had twins

None of the verified UK sources gives a recurrence figure, so we are not going to invent one. What is worth knowing is that the factors above do not disappear after a twin pregnancy: an inherited tendency to release more than one egg is still an inherited tendency, and age keeps moving in one direction. Ask your GP or midwife rather than a forum.

The number that matters more than the odds

Once twins are confirmed, the interesting statistic stops being how you got here. NICE NG137 is explicit that the risks associated with twin and triplet pregnancy are determined by chorionicity and not zygosity, meaning how many placentas and sacs your babies have, not whether they came from one egg or two. Two babies with a placenta each are monitored differently from two babies sharing one. That distinction is made at the first scan and drives everything afterwards.

So by all means work out the odds beforehand. Afterwards, the only question worth asking your sonographer is which type of twin pregnancy you have.