It’s very common to hear that twins ‘run in the family’. That is often true! Studies have shown that a woman who has a mother or sister – who has birthed twins – is twice as likely to also have twins. However, twin pregnancies can happen by chance and not always because of genetics. So how rare are fraternal twins?

As of 2003, the average of twins that were born was about 16 sets per 1,000 births in the U.S. While the ratio of natural twin pregnancies is estimated to be 1 in 250 pregnancies. Many factors can play into increasing the chance of twin births naturally. Also, fertility technology plays a role in the number of twin pregnancies, overall increasing the chances. If you’re wondering how rare fraternal twins are, they are the more common type of twins compared to identical twins which contributes to the overall frequency of twin births.

Types Of Twins

There are two types of twins: Identical and Fraternal.

Identical twins are just that, two babies who look identical to each other. Identical twins form from one single egg fertilized by one single sperm that ends up dividing into two separate embryos early on in the pregnancy. Considering both babies came from the same cell, they will share the same DNA. This is often what makes them look identical.

The chances of having identical twins are relatively rare. For every 1,000 births, identical twins happen every 3rd or 4th. Although, the use of fertility treatments will increase the risk of having identical twins.

Identical twins make up 0.95% of pregnancies that were conceived with treatment. The reasoning for this is unclear. With the advancement in more accurate IVF technology, the birth rate of identical twins due to infertility treatments has declined over time.

Fraternal twins are when two different sperm fertilize two different eggs. This is possible when a woman produces two eggs at the same time, called hyperovulation. Unlike identical twins, fraternal twins don’t share the same DNA and they don’t necessarily end up being the same sex. They may or may not look alike and can be a combination of both boys and girls or two girls and two boys.

How Common Are Twins?

In 2019 in the United States, CDC birth statistics found that there were 120,291 infants born as twins. That’s an average of 32.1 per 1,000 live births. This number includes naturally occurring multiples and women who went through fertility treatments.

In the 1990s, multiple pregnancies peaked and have been declining since with the advancement of technology and fertility treatments.

Factors That Can Increase Your Odds Of Having Twins

Fertility treatments aren’t the only factors that increase the chance of twin pregnancies. There are other factors that can contribute to higher odds of specifically fraternal twins. But just how rare are fraternal twins? While fraternal twins are more common than identical twins, they still represent a relatively small percentage of all births. Below, we look into some of the factors that play a part in increasing the chance of twin pregnancies.

Genetics

Genetics for hyperovulation can be inherited. If fraternal twins run on a woman’s sister’s side, she is 2.5 times more likely to have twins herself. If a woman’s mother has fraternal twins, she is about 2 times more likely to have twins. The chances are higher when a woman has a mother who has fraternal twins in her family.

Her DNA can influence how many eggs the mom can release. Interestingly enough, this statistic doesn’t apply to the mother-to-be if her partner’s family has any twins. This will not increase her chances of having twins.

Age

Generally older women have a higher chance of conceiving twins than younger women. A woman at 35 years old is 4 times more likely to have fraternal twins. As women age near menopause, they will experience hormonal changes where their follicle-stimulating hormone (FSH) rises. Because their FSH is responsible for the development of eggs in the ovaries, this rise in stimulation to grow could encourage their body to release more than one egg during ovulation.

Height

Women who are taller than average are more likely to have twins. Research has shown that women who are at least 5.4” tall are about 1.5-2 times more likely to have a twin pregnancy. The reason behind it isn’t fully understood but the theory is the relation to taller women and the production of a certain protein in a higher quantity.

Once this protein gets released as a response to growth factor, this can make the ovaries more sensitive to the follicle-stimulating hormone. This may increase the rate of twin pregnancies.

Weight

Women who have a higher body mass index than 30.1 are more likely to conceive twins. When a woman is carrying extra body fat, it can lead to higher levels of estrogen. Higher estrogen can mean an overstimulation of the ovaries. This means the ovaries may release two or more eggs at once, increasing the odds of twins.

Diet

Women who eat more dairy than the average show that the odds of a multiple pregnancy are enhanced. Sounds a little exaggerated but it can actually make it up to five times more likely. This is because of the growth hormone given to cows – which affects the hormone levels in humans.

Ancestry

If a woman’s family has a history of twin sons on her mother’s side, this might mean a higher likelihood of a twin birth. However, the offspring of male identical twins may have a higher chance of having their own identical twins. If both parents have a history of fraternal twins, the chances of conceiving twins also increase.

Previous Birth

If a woman has a child before, that pregnancy can also increase the chances of having twins.

In Vitro Fertilization

An increased use of assisted reproduction treatments has been used over the past few years. In vitro fertilization (IVF), ovulation induction, and intra-uterine insemination are some of the causes of the increase in twin births.

How Common Are Twins With Treatments?

Conceiving multiples is a potential risk of fertility treatments. Specifically, treatments that can lead to multiples, twins, triplets, etc. Conceiving twins is considered a risk due to the added risk to the parents and babies. Careful monitoring and targeted treatment can reduce the risk of a multiple pregnancy. However, not all treatments increase your odds but a lot of them do. Some of the treatments that may lead to twins are:

• Clomid (clomiphene)
• Femara (letrozole)
• Gonadotropins (also known as injectables) like Gonal-F and Follistim
• IUI treatment, when used along with fertility drugs
• IVF treatment

If you happen to check several of the boxes above, your chances of having twins increase even more slightly.

The Bottom Line When It Comes To Twins

The chances of a woman having twins is one out of 350 to 400. One-third of those numbers will be identical twins. The reality is that the commonality of conceiving twins is complex. Being able to predict if a woman will or will not have twins isn’t precise. In the end, everyone has a chance to conceive twins, some people just happen to have higher chances than others.

If You Have More Questions About Twins Pregnancies Call Rosh Maternal and Fetal Medicine In NYC

The doctors at Rosh Maternal & Fetal Medicine specialize in obstetrics. We’re trusted experts in multiple pregnancies and will provide the highest quality of care with holistic prenatal care and all the advanced medical procedures you may need.

The doctors at Rosh Maternal & Fetal Medicine have extensive experience determining the best plans for your pregnancy. If you want more information about twin pregnancies or “How rare are fraternal twins?”, call our office in the Midtown East area of New York City, or schedule an appointment online.