12 Things You Didn't Know About Twins and Multiples

Did you know that pregnancy with multiples happens more often now than in the past? It's probably because two factors that increase the odds of multiples—fertility treatments and getting pregnant after 30—are becoming more widespread. So how common are multiples, and what can you expect throughout the nine-month gestation? 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 31.1 out of every 1,000 live births. Parents give birth to triplets or higher-order multiples in 79.6 out of every 100,000 live births. And while more multiples are born now than in past decades, the number is slowly starting to decline—possibly 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 womb simultaneously. They share the same amount of genetic material as other full siblings.

On the other hand, identical (monozygotic) twins develop from a single fertilized egg, which is rare. They might share a placenta or amniotic sac and have the same DNA. 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 than that? Here's the proper terminology.

Multiples Terminology
Terminology	Number of Babies
Twins	2
Triplets	3
Quadruples	4
Quintuplets	5
Sextuplets	6
Septuplets	7
Octuplets	8
Nonuplets	9
Decuplets	10

4. Multiples Have a Variety of Causes

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

5. Most Multiple Births Used To Occur in the Northeast

Historically, there was a time when the highest incidence of twins 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

In May 2021, news broke that a 25-year-old Malian woman gave birth 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. Multiple Pregnancies Are Often 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. But don't worry: If you're pregnant with multiples, your doctor will carefully monitor your pregnancy, and they'll often recommend regular ultrasounds and check-up appointments.

8. Multiples Are Usually Delivered at 37 Weeks or Earlier

Why? When a person is pregnant with multiples, an earlier delivery date decreases the risk of stillbirth or other complications. And you don't need to worry about a drawn-out delivery: Research shows that the average birth time between twins is around 15 minutes.

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

Most multiple pregnancies end in C-section births—especially if you're having triplets or more. 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 natural birth 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. And sometimes, 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, 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.

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

In a study published in the Journal of Reproductive Medicine, researchers found that the average height of birthing parents carrying multiples (twins or triplets in this case) was 5'5". In contrast, the national average height for adult females is 5'3.75".

A separate study by the same research team, also 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.

12. Multiples Can Get Absorbed in the Womb

This might seem gruesome, but "vanishing twin syndrome" actually occurs in 20–30% of multifetal pregnancies. It happens when one baby stops developing, and it's usually absorbed back into the uterus. Most cases occur before the doctor even hears a fetal heartbeat.