12 Things You Didn't Know About Twins and Multiples

Whether you are having twins or simply curious about multiples, there is just something fascinating and exciting about pregnancies with more than one baby. But even as these births and sets of babies get lots of attention, many people don't know a lot about them. For instance, did you know that pregnancy with multiples happens more often now than in the past?

This change is due to the fact that two factors that increase the odds of multiples—fertility treatments and getting pregnant after 30—are becoming more widespread. So, how common are multiples, what is the record for the most babies born at once, and what can you expect throughout the nine-month gestation with multiples? Keep reading to learn more about twins, triplets, and higher-order multiples.

1. Twins Are the Most Common Type of Multiple Pregnancy

According to the Centers for Disease Control and Prevention (CDC), twins occur in 31.2 out of every 1,000 live births. Triplets or higher-order multiples account for 80 out of every 100,000 live births.

Interestingly, while significantly more multiples are born now than in past decades, the number is slowly starting to decline. This change is happening in part because it's becoming less common to get fertility treatments involving the transfer of multiple embryos, according to a study published by the CDC.

2. Identical Twins Share All the Same Genetic Material, but Fraternal Twins Don't

Fraternal (dizygotic) twins happen when two separate eggs are fertilized by two different sperm, and they implant in the uterus simultaneously. They share the same amount of genetic material as other siblings who share the same biological parents. In other words, they are no more similar than siblings born at different times. Additionally, fraternal twins can be the same sex or different sexes.

Identical (monozygotic) twins, on the other hand, develop from a single fertilized egg. This type of twin is rare. They might share a placenta or amniotic sac and have the same DNA. They are either both male or both female. That said, because of environmental factors like their position in the womb, identical twins won't look 100% alike, according to the American Academy of Pediatrics (AAP).

3. There Are Medical Terms for Multiple Births

Most people know that "twins" means two babies and "triplets" means three babies. But what if you're expecting more? Here's the proper terminology.

4. Multiples Have a Variety of Causes

Factors that increase the likelihood of pregnancy with multiples include maternal age greater than 30 (hormonal changes make the release of more than one egg at ovulation more common), a history of multiples in your family, and a previous pregnancy with multiples. Fertility drugs and assisted reproductive technologies, such as Clomid (clomiphene) and in vitro fertilization (IVF), also raise the odds of fraternal twins because they lead to the production of more than one egg or embryo.

5. Most Multiple Births in the US Used To Occur in the Northeast

Historically, there was a time when the highest incidence of twins in the US happened in Connecticut, Massachusetts, New Jersey, and the District of Columbia. This may have been because parents in these locations tended to have babies later in life and were more likely to use fertility treatments.

However, those numbers have been shifting through the decades. Statistical analysis shows shifting trends reflecting that twins are just as likely in Michigan, Alabama, and Iowa as in Maine, Connecticut, and Massachusetts.

6. The Largest Multiple Birth Happened in 2021

The record for most babies born at once in a single birth sits at nine. In May 2021, news broke that a 25-year-old Malian woman gave birth in Morocco to nine babies (nonuplets), even though she was expecting seven. She ended up breaking a world record for multiple births. Previously, the record for most children delivered in a single birth to survive was held by American Nadya Suleman (known as Octomom by the media), who gave birth to eight kids in 2009.

7. Pregnancies With Multiples Are Considered 'High Risk'

Multiple pregnancies come with an increased risk of preterm labor, uneven growth, high blood pressure, preeclampsia (a condition characterized by high blood pressure and protein in the urine), diabetes, and other "high-risk" pregnancy conditions. These risks increase for those pregnant with triplets or other higher-order multiples.

But don't worry: If you're pregnant with multiples, comprehensive prenatal care can help mitigate these risks. Your doctor will carefully monitor your pregnancy, and they'll often recommend regular ultrasounds and check-up appointments.

8. Multiples Are Usually Delivered at 38 Weeks or Earlier

When a person is pregnant with multiples, an earlier delivery date decreases the risk of stillbirth and other complications. Research shows that the optimal timing of delivery is at 38 weeks. However, multiples are often delivered at 37 weeks or sooner due to complications like preterm labor.

One plus is that you don't need to worry about a drawn-out delivery: Research shows that the average birth time between twins is around 15 minutes. However, in extremely rare cases, when one twin is delivered prematurely but the other continues to thrive in the uterus, the second baby may be delivered a month or more later!

9. You're More Likely To Have a C-Section With Multiples

Most multiple pregnancies end in C-section births—especially if there are three or more babies. That said, you could be a candidate for a vaginal delivery if you're having twins and they're both positioned with their heads down.

Your doctor might also consider vaginal birth a safe option if Baby A (the one closest to your cervix) is head down and Baby B is breech, as long as they're about the same size. Other times, one twin is born vaginally, and the other is born via C-section; this is most common if Baby B shows signs of distress.

10. Multiples Socialize With Each Other in the Womb

Here's an endearing fact about multiples: A novel study published in PLOS one detailed research involving five pairs of twins. After studying them with ultrasound technology during gestation, experts determined that the twins were physically contacting each other.

"We conclude that performance of movements towards the co-twin is not accidental: already starting from the 14th week of gestation twin foetuses execute movements specifically aimed at the co-twin," the study reads.

Additionally, research shows that co-sleeping in the neonatal nursery promotes healthy growth and development in preterm twins.

11. Birthing Parents of Twins Are More Likely To Be Tall and Eat Dairy

Research shows that taller parents are more likely to have twins than their shorter counterparts. People with a higher body weight are also more likely to get pregnant with twins and other multiples.

A 2006 study published in the Journal of Reproductive Medicine, concluded that birthing parents who consume animal products (specifically dairy) were five times more likely to have a multiples birth compared to vegan birthing parents. Additionally, note that prospective parents who are on vegan and vegetarian diets (especially those who abstain from dairy) have reduced rates of conception overall.

12. Multiples Can Get Reabsorbed in the Uterus

This might seem gruesome, but "vanishing twin syndrome" actually occurs in 20% to 30% of multifetal pregnancies. This happens when one fetus stops developing, and it's usually absorbed back into the uterus. While it can't be prevented, most cases occur early in the first trimester, before embryonic cardiac activity is detected, and the other baby can continue developing normally.