How common are twins and other multiple births?

As of 2013, twins accounted for about 3 in 100 births in the United States. And 1 in 837 births were triplets or higher order multiples.

Between 1980 and 2009, the birth rate for twins rose by about 76 percent and has remained about the same since then. The rate of triplets and higher order multiples quadrupled from 1980 to 1998 but has since dropped by more than one-third.

It's possible that the birth rate of fraternal twins, which happens when two eggs are fertilized by two different sperm, has slowed because of counseling by fertility doctors about the health risks of having twins or more. (The likelihood of having identical twins, which happens when one fertilized egg divides in half, is holding steady at about 3 to 5 in 1000 births. This rate hasn't changed over the decades and is remarkably constant all over the world.)

What caused the increase in multiples?

It was partly due to more women waiting longer to have a baby. Hormonal changes as you age make it more likely that your body will release more than one egg at a time – and more than one fertilized egg often means more than one baby.

But the rise in multiple births was mostly due to women using fertility drugs and assisted reproductive technology (ART) to help them conceive. These drugs and certain treatment decisions greatly increase the odds of having twins or higher order multiples.

If I'm undergoing fertility treatment, how likely am I to have more than one baby?

It depends on the type of fertility treatment.

Fertility drugs stimulate your ovaries, increasing the odds that you'll release several eggs at the same time.

If you have in vitro fertilization (IVF), your chances of having more than one child vary depending on your age, how many embryos are placed in your uterus, and other factors. According to national data, the percentage of IVF treatments resulting in twins or more were as follows:

29.1 percent for women younger than 35

26.5 percent for women age 35 to 37

20.9 percent for women age 38 to 40

13.6 percent for women age 41 to 42

8.8 percent for women age 43 and above

On its own, IUI (intrauterine insemination), the procedure in which sperm are placed into the uterus with a syringe, is the only fertility treatment that doesn't increase the chances of conceiving multiples. But most women who undergo IUI also take a fertility drug, which does.

What other factors affect the chances that I'll have more than one baby?

Although conceiving identical twins almost always happens by sheer chance, several factors can influence the probability that you'll have fraternal twins.

Heredity: If you're a twin, or if there are twins in your family, you're slightly more likely to have a set yourself.

If you're a twin, or if there are twins in your family, you're slightly more likely to have a set yourself. Age: The older you are, the higher your chances of having fraternal twins or higher order multiples. Women age 35 or older produce more follicle-stimulating hormone (FSH) than younger women. FSH is also the hormone that causes an egg to mature in preparation for ovulation each month, and women with extra FSH may release more than one egg in a single cycle. So older women are statistically less likely to get pregnant, but they're more likely to have twins if they do.

The older you are, the higher your chances of having fraternal twins or higher order multiples. Women age 35 or older produce more follicle-stimulating hormone (FSH) than younger women. FSH is also the hormone that causes an egg to mature in preparation for ovulation each month, and women with extra FSH may release more than one egg in a single cycle. So older women are statistically less likely to get pregnant, but they're more likely to have twins if they do. History of twins: Once you have a set of fraternal twins, you're twice as likely to have another set in future pregnancies.

Once you have a set of fraternal twins, you're twice as likely to have another set in future pregnancies. Number of pregnancies: The more pregnancies you've had, the greater your chances of having twins.

The more pregnancies you've had, the greater your chances of having twins. Race: Twins are more common than average among white and African American people and less common amon Hispanic and Asian people.

Twins are more common than average among white and African American people and less common amon Hispanic and Asian people. Body type: Fraternal twins are more common in large and tall women than in small women.

Find out more:

When and how to find out if you're having twins or more

Pregnant with multiples: Prenatal care

Twins fetal development in images