The short answer
Fraternal twins can run in families. Identical twins mostly do not.
The difference comes down to how each kind of twin starts. Fraternal twins need two eggs released at the same time, and the tendency to do that can be inherited. Identical twins start from a single egg that splits, which seems to happen largely by chance.
Identical and fraternal twins
Identical (monozygotic) twins start from one fertilised egg that splits in two. They share the same genes and are always the same sex.
Fraternal (dizygotic) twins, also called non-identical twins, start from two separate eggs that are both fertilised. They are no more alike than any other two siblings, and can be the same sex or different sexes.
According to the NHS, about one third of twins are identical and two thirds are fraternal.
Why fraternal twins can run in families
Some people tend to release more than one egg in a cycle. That tendency can be inherited, and it is linked to the parent whose body releases the egg. A history of fraternal twins in that parent's family is what raises the odds.
A large review of research on fraternal twinning found that mothers of fraternal twins report more female relatives with fraternal twins than mothers of identical twins do. Researchers have started to find genes involved, but the variants found so far are rare and explain only a small part of it.
The other side of the family is not always irrelevant. A 1983 study of the families of 950 twin pairs also found that the family of the parent who provides the sperm plays some part in fraternal twinning. For your own pregnancy, though, it is the family of the person who ovulates that counts most.
Identical twins mostly happen by chance
The NHS says identical twins do not run in families, and that everyone has about the same chance of having them: about 1 in 250. Across the world, identical twins make up about 4 in every 1,000 deliveries, a rate that stays fairly constant from country to country.
The picture is not completely settled. The same 1983 study found signs that a tendency to identical twins can also be passed down in some families. For most people, though, identical twins are a surprise.
What raises the chance of twins
Apart from family history, these factors make fraternal twins more likely:
- Age. Over 35, you are more likely to release more than one egg when you ovulate.
- A previous twin pregnancy. People who have already had twins are more likely than average to have twins again.
- Earlier births. The chance of fraternal twins goes up with the number of children you have already given birth to.
- Height and weight. People who are taller, or who have a body mass index above 30, have a higher chance of fraternal twins.
- Fertility treatment. Drugs that boost ovulation and IVF both raise the chance of twins. With IVF, the chance goes up when more than one embryo is transferred. Many people now have a single embryo transferred, which considerably lowers the chance of a multiple birth.
- Background. Fraternal twins are more common in some ethnic groups. The NHS notes the highest rate among Nigerians and the lowest among Japanese people.
None of these guarantees twins, and there is no guaranteed way to make identical twins more likely.
Twin rates differ between countries
Twins are more common than ever. A study of 165 countries found that the global twinning rate rose by a third between the early 1980s and the early 2010s, from 9.1 to 12.0 twin deliveries per 1,000 deliveries.
In 2010 to 2015, Africa had the highest rate, 17.1 per 1,000, with North America close behind at 16.9 and Europe at 14.4. Asia (9.2) and South America (9.3) had the lowest. Because identical twins happen at a similar rate everywhere, most of these differences come from fraternal twins.
The researchers put much of the recent rise down to the growth of fertility treatment, and partly to people having children later in life.
If you are expecting twins
Twins can usually be seen at the 12-week scan, where you should also be told whether the babies share a placenta. Carrying twins brings extra risks: the HFEA notes that at least half of twins are born premature and underweight. Your midwife or doctor will explain what this means for you and what care to expect.
The due date calculator and pregnancy week calculator are based on the usual 40 weeks, so treat them as a starting point and go by the dates your midwife or doctor gives you. If you are still trying, the ovulation calculator shows your most fertile days, though it cannot tell you how many eggs you will release.
And if you are simply wondering what twins might look like, the twins generator can make a picture of possible twins from your two photos, just for fun.