What are my chances of having twins?
A curious adult from Colorado asks:
“My father is an identical twin and my husband's grandfather is a fraternal twin. Does this increase our chances of having twins, or is the genetic trait only on the mother's side and only for fraternal twins? (P.S. we've always hoped for twins - a boy and a girl.)”
As a twin myself, I've often wondered the same thing. From what I've read, you're right on both counts. Identical twins do not run in families and a history of fraternal twins only helps if it comes in on the mother's side.
Because fraternal twins happen by mom releasing two eggs, the dad's side of the family doesn't come into play. (Well, at least not until his daughters have kids...)
A woman who is herself a fraternal twin is 2.5 times as likely to have twins as someone from the general population.1 A mother of fraternal twins is 3-4 times more likely to have another set of fraternal twins.2 A woman who is an identical twin is no more likely to have twins compared to someone else.
Types of Twins
Why do fraternal twins run in the family but not identical twins? It makes sense once we understand the differences between the two types of twins.
Identical twins come from the same fertilized egg and are sometimes called monozygotic. What this means is that they share all the same genes and DNA, which is why they can be so hard to tell apart! (Amazingly, though, their fingerprints are different). The frequency of identical twins is the same everywhere, about four in every 1000 births.3
How can they come from the same egg? Sometime before the 8th day after the egg and sperm meet, the fertilized egg splits into two. These two "halves" then grow into identical twins.
This is different from what happens with fraternal twins. Fraternal twins come from two different eggs and are sometimes called dizygotic. They are really like any two siblings who happen to be born at the same time.
What happens with fraternal twins is that the mom releases more than one egg at a time. (In other words, she ovulates more than once per menstrual cycle). Twins result from two eggs getting fertilized and growing in the uterus at the same time. Around twelve pairs of fraternal twins are born every 1000 births.2
Because they come from different sperm-egg pairs, they don't look any more alike than any other siblings. This is also why half of fraternal twins are a boy and a girl.
Chances of Having Twins
OK, so what does this have to do with genetics? Fraternal twins result from two ovulations, while identical twins result from the splitting of one embryo into two. Ovulation is a normal process that involves many genes. Embryo splitting, on the other hand, appears to occur randomly by chance.
In other words, your genes can affect whether you have fraternal twins because genes are involved in the process of ovulation. Identical twins are random and so genes don't have much influence at all.
The genes that cause a woman to have fraternal twins are unknown. One theory is that hormones are important. For example, the hormone FSH or follicle-stimulating hormone may be higher in mothers of twins.
FSH is necessary for egg growth and is a commonly used fertility drug. Mothers of fraternal twins tend to be taller, and have earlier and shorter menstrual cycles. These traits could also be due to higher hormone levels.
What else can affect whether you'll have fraternal twins? Ethnic background is one (although this is based in genetics as well). For example, a woman of African origin is twice as likely to have twins compared to her Caucasian friend. She is four times as likely as her Asian friend.2
Other factors that can increase the chances of having a fraternal twin are:
Mother's age. A 35-year-old mother is four times more likely than a woman under the age of 20 to have twins. After age 35, however, the chance of having twins naturally decreases.2,3
Nutrition. Women who eat poorly are less likely to have twins. Tall and overweight women, on the other hand, are more likely to have twins.2
Previous births. Doesn't everyone know someone who only wanted two kids but ended up having twins for the second pregnancy? This is because the twin rate goes up after a previous birth.4
Fertility drugs. Different treatments will vary, but on average, 20% of live births after fertility treatments are fraternal twins.2 (This is why women over the age 35 actually have a high chance of having twins; they are much more likely to use fertility drugs).
Unfortunately, twins running on dad's side of the family don't increase your chances much at all. Good luck, though!