You’ve heard the news that Beyonce’s expecting two babies—but you may not realize there’s more to twins than fraternal versus identical.
This article originally appeared on health.com.
Beyoncé is pregnant with twins! The singer and actress, 35, announced the news on Instagram today, posting a photo of her baby bump and writing that she and husband Jay Z “have been blessed two times over.”
Pregnancies with multiple babies aren’t as rare as they once were, but they’re still an uncommon event. So in celebration of the couple’s happy news, here are 10 fascinating things you may not know about twins—identical, fraternal, and otherwise.
Twin births are on the rise
There were 135,336 twin births in the United States in 2014, the latest year for which data is available. That means about 3.4% of all babies born that year were twins—a record high, according to the United States Centers for Disease Control and Prevention.
The twin birth rate has been climbing steadily since 1980, and experts say that the prevalence of in vitro fertilization (IVF) and other assisted reproductive technologies has likely played a role.
Older moms are naturally more likely to have twins
Even without IVF or other fertility treatments, mothers are naturally more likely to have twins as they get older, says Christine Greves, MD, an obstetrician and gynecologist at Winnie Palmer Hospital for Women and Babies in Orlando.
“It may have to do with evolution,” says Dr. Greves. “Women at an older age won’t be able to have babies for much longer, so it makes sense that they’d have more at one time.”
Twins live longer than singletons—and so do their moms
In a 2016 study from the University of Washington, researchers found that identical twins tend to live longer than fraternal twins, and all twins tend to live longer than the general population. "There is benefit to having someone who is socially close to you who is looking out for you,” said study author and postdoctoral researcher David Sharrow, PhD, in a press release. “They may provide material or emotional support that leads to better longevity outcomes.”
There’s good news for moms of twins, as well: In 2011, University of Utah researchers discovered that women who give birth to twins tend to live longer than other mothers. (It’s not because having twins makes you healthier, say the researchers, but that healthier women are more likely to have twins.)
There are more than two types of twins
Everyone knows the difference between identical and fraternal twins, but those aren’t the only possibilities. Some siblings are mirror-image twins, with birthmarks on opposite sides of their bodies, for example.
Twins can also be conceived—and born—up to 24 days apart, when a woman ovulates more than one egg in a cycle but the eggs are released at different times. Last year People reported on an Australian woman who experienced this extremely rare phenomenon, known as superfetation or interval birth.
Twins can have different skin colors—and even different fathers
Twin girls born to Whitney Meyer and Thomas Dean in 2016 were far from identical: One has light skin like her Caucasian mother, and one has a darker complexion like her African American father. “We don’t know how often it happens because not all cases come to our attention,” Nancy L. Segal, PhD, director of the Twin Studies Center at California State University, told People.
Babies born at the same time to the same mom can even have different dads. One of these rare cases, which occurs when a woman has sex with two different men during her ovulation window, occurred last year in Vietnam.
There’s a gene for fraternal twins
It’s often assumed that twins run in families, but so far research only shows a link between genetics and fraternal twins. A 2016 Dutch study identified two gene variants that increased a woman’s odds of having twins by 18% and 9%; if women had both variants, it increased their odds by 29%.
Twins are more likely to be left-handed
About one in five twins, both fraternal and identical, are left-handed, according to a 1996 Belgian study. That’s about double the rate in the general population.
Tall women are more likely to have twins
A 2006 study found that, on average, mothers of multiples tended to be about one and a quarter inches taller than mothers who had only single births. The study authors speculated that taller women tend to have higher levels of insulin-like growth factor—a protein released by the liver that’s been linked to height as well as twin births.
Twins should be delivered at 37 weeks
To reduce the risk of stillbirth and complications, experts say that the best time to deliver twins is at 37 weeks gestation—or 36 weeks, if the babies share a placenta.
For a lot of twin pregnancies, early delivery happens naturally. “A lot of twins don’t make it to 39 or 40 weeks full-term, because the mother’s uterus is stretched significantly more at an earlier point in the pregnancy,” says Dr. Greves. This also puts twin pregnancies at an increased rate of pre-term labor (before 37 weeks).
C-sections aren’t the only delivery option
C-sections are more common with twin deliveries than with single babies, but vaginal births can be safe, as well, says Dr. Greves. “If both heads are down and the babies are cooperating and the placenta’s in the right position, we love to try for vaginal delivery,” she says.
After the first baby is delivered, Dr. Greves says the second usually comes along quickly—although that’s not always the case. “There have been times when I’ve waited a few hours for the second baby,” she says, “but usually it’s less than an hour.”