How the calculator works
For a natural pregnancy, the calculator starts from real births: how many maternities in England and Wales were twins in 2025, for each age group of the mother, from the Office for National Statistics. Across all ages it was 14.3 in every 1,000, about 1 in 70.
It then splits that figure in two. Identical twins happen at about the same rate everywhere and at every age, about 4 in every 1,000 births, which the NHS puts as about 1 in 250. Everything above that is fraternal twins, and that is the part that changes with age and with family history.
If you tick that fraternal twins run in the family of the person who ovulates, or that you have had twins before, the calculator doubles the fraternal part. It does not double it twice when both are true, because both point to the same tendency.
Identical and fraternal twins
Identical twins come from one fertilised egg that splits in two. They share the same genes and are always the same sex. Why an egg splits is not well understood, and it seems to happen mostly by chance.
Fraternal twins, also called non-identical twins, come from two eggs released in the same cycle and fertilised separately. They are no more alike than any other brothers and sisters, and can be a boy and a girl. According to the NHS, about one in three twin pairs is identical and two in three are fraternal.
Why age matters
Releasing two eggs in one cycle becomes more common as people get older, up to the late thirties. In England and Wales in 2025, the twin rate was 6.8 per 1,000 maternities under 20, 12.2 at 25 to 29, 14.7 at 30 to 34 and 17.0 at 35 to 39. A review of research on fraternal twins describes a fourfold rise from age 15 to 35.
These national figures include pregnancies after fertility treatment, which become more common after 35. For a natural pregnancy at 35 to 44, the calculator's figure may therefore be a little high. At 45 and over, most twin births follow egg donation or IVF, so the calculator does not give a natural estimate.
Twins in the family
A tendency to release more than one egg can be inherited, and it matters on the side of the person who ovulates. In a large Australian twin register, sisters of mothers of fraternal twins had 1.7 times the usual chance, and daughters of female fraternal twins 2.5 times. Older studies found about twice the chance for mothers and daughters. Twins in the partner's family do not change the chance for this pregnancy.
Having had fraternal twins before points the same way: a Swedish study found the chance of twins again in a later birth was two to three times higher. Identical twins mostly do not run in families. Our guide to twins in the family goes into more detail.
Twins after fertility treatment
Fertility treatment changes the picture so much that the calculator shows its own figures instead of adjusting the natural ones:
- IVF with one embryo: about 1 to 3% of births are twins in UK clinics. Twins only come if the embryo splits.
- IVF with two embryos: over 30% of births were twins for patients under 35 in UK clinics, falling with age to under 15% after 44.
- Insemination (IUI): in Europe, about 8% of births with a partner's sperm and about 6% with donor sperm were twins.
- Ovulation tablets such as clomiphene or letrozole: 3% to 13% of pregnancies were twins in two large trials.
- Ovulation injections (gonadotropins): in a large trial, 32% of pregnancies were twins or triplets.
Your clinic knows your own treatment and can give you a more precise figure.
What the calculator leaves out
Some studies link taller height, a higher BMI or having had several children to a slightly higher chance of fraternal twins, but the results are small or inconsistent, so the calculator does not use them. Diet, supplements and the timing of conception have no proven effect. The country you live in matters a little: twin rates are similar across western Europe, between about 14 and 16 per 1,000 maternities.
If you are expecting twins
Twins usually show up at the first scan, around 12 weeks, which also shows whether the babies share a placenta. A twin pregnancy needs closer care, because twins are more often born early and small. Your midwife or doctor will explain what that means for you. The due date calculator still works as a starting point, though twins often arrive before the due date.
And if you are simply curious what two little faces of yours might look like, the twins generator makes a picture of two possible children from a photo of each of you, just for fun.