Do twins skip a generation?

We don’t know. Yet it may seem so.

Let’s say the hyperovulation theory is at work, and the man inherits the hyperovulation gene. That gene won’t help him have twins (his partner should have that gene), but if he passes that gene on to his daughters, they have an increased chance of having fraternal twins. So it might look like this:

• Generation 1: The mother has the gene for hyperovulation and has fraternal twins, a boy and a girl. Both inherit the gene for hyperovulation.
• Generation 2: A female twin has a set of twins. One of them inherits the gene for hyperovulation. A male twin does not have twins, but passes the gene for hyperovulation on to his daughter.
• Generation 3: Daughters who have inherited the hyperovulation gene give birth to twins.

However, genetics is not always as simple as this example. Again, some genes are dominant while others are recessive, so – if the gene for hyperovulation is recessive – you would have to inherit two recessive genes to have the trait. Additionally, we don’t know if the gene needs something in the environment to activate it.

Is having identical twins genetic?

Geneticists used to think that identical twins don’t live in families, because the splitting of a fertilized egg happens randomly. Although research has shown that most cases of identical twins are not genetic, there are families with a higher than normal number of identical twins, which means there may be something genetic going on.

One explanation is that identical twinning can occur because of a mutation in a gene or genes that control how the cells of early embryos stick together. (An embryo with this gene may not attach well, which could cause it to split before it implants.)

Which side of the family do the twins come from?

The gene for hyperovulation can come from either side of the family, but a woman would have to have the gene for a couple to have twins based on genetics. In other words, having twins on your father’s side of the family will have no effect on your chances of having twins.

It’s possible, however, that if dad inherited the gene for hyperovulation, he could pass the gene on to his daughters, increasing their chances of having twins—and your chances of having twin grandchildren.

But remember, the hyperovulation theory has yet to be proven.

What other factors contribute to the birth of twins?

Many factors besides genetics can affect the possibility of having fraternal twins, such as:

• Mother’s age. The older the age group, the higher the rate of multiple births. The average birth rate of twins is 32 per 1,000 births. Women aged 45 to 54 have a higher twin birth rate of 107 per 1,000, while women aged 15 to 19 have a twin birth rate of 16 per 1,000.
• Ethnic background. Fraternal twins are more common in black populations and less common in Asian and Hispanic populations.
• Body composition. Taller mothers (65 inches or more) and mothers who are obese (BMI of 30 or more) are more likely to have fraternal twins than shorter women (less than 61 inches) and women who are underweight (BMI less than 20) . This may be because taller and heavier people have more insulin-like growth factor (IGF), which is associated with increased ovulation.
• Number of pregnancies. The odds of having twins increase with each pregnancy, even after adjusting for maternal age.
• Fertility treatments. Both fertility drugs and in vitro fertilization (IVF) can result in fraternity. In fact, fertility treatments help explain the skyrocketing number of fraternal twins (and multiples) born in recent years. Fertility drugs stimulate ovulation, often more than one egg at a time. If more than one is fertilized, multiples can occur. During IVF, eggs are removed from the ovaries, fertilized with sperm and transferred back into the woman’s uterus. In the past, more than one embryo was transferred back (to increase the chances of success). This sometimes resulted in twins. However, this is much less common now, and in most cases single embryo transfer is preferred, due to the higher risk of pregnancy complications when multiple births occur.
• Time of year. The fewest twins were conceived in January, and the most in July. This may be because the length of daylight affects the secretion of follicle-stimulating hormone (FSH), which stimulates ovulation.

Think you’re having twins? A first trimester ultrasound will confirm that it is, but there are some early signs of a twin pregnancy that might make you wonder. Read our article on signs you are having twins or multiples to learn more.