Think raising a singleton is tough? How about raising two babies at one time? Twins can be tough to carry and tough to raise, and there’s still a lot more than experts are finding out about how twins work – both in the womb and out. In fact, some recent studies are showing that older women are more likely to carry twins, twins may have a higher chance of becoming lefties, and that they may have their own special language that others just don’t understand.

People may think they know all about twins – especially parents who have them – but unless they’re a twin themselves, there’s probably a lot they still don’t understand. Just how common is it for parents to mix up their twins? Can twins have different dads? Is it possible for twins to be born in different years? Most people will likely be surprised with the answers.

But, as twins become more common (that’s another fact, according to several sources that say twin births are on the rise), it’s important to learn as much as possible about them, from what twin pregnancies are like and what it’s like parenting on double duty. These fun, incredible facts about twins may be surprising, but they also make them even more intriguing.

15 Likelier With Age

If you’re a woman over 30, be prepared: you’re more likely to have a twin pregnancy than younger women. If you’re closer to, or over, 40, your chances are even higher. Older women have a lot of hormonal changes going on that may increase the likelihood of conceiving twins naturally due to an increased ability to ovulate with two eggs at a time. Conception may be more difficult as you get older, but when you do conceive, you may end up with twins.

Higher levels of the hormone, FSH, can also play a role. This hormone stimulates the follicles that prepare and release eggs, and as you get older, this hormone increases. So, if you go for your first ultrasound and see two beautiful bundles of joy baking in your belly, rejoice – and then blame it on the hormones!

14 More Babies, More Problems

If you thought morning sickness was bad with a singleton, you should really empathize with mommies of twins. Although it’s not the case for every twin mommy, experts say that morning sickness may be worse when you’re carrying twins. Morning sickness itself is still one of those things that remains somewhat of a mystery and can be caused by several things. One of those things could be an increased hormone known as human chorionic gonadotropin.

This hormone is higher during pregnancy and may double for twin pregnancies, which of course, may lead to increased morning sickness. Fortunately, even women pregnant with twins can usually find some relief after the 12-week mark like moms of singletons, when the levels of the hormone tend to regulate and the first trimester comes to a close.

13 Not Part Of The Circle

If you’ve ever been around twins and notice that they seem to have their own special language – sometimes communicating without even speaking a word to each other – you may not be imagining it. Even those closest to twins, like parents or other siblings, may not be “special” enough to understand the special bond that twins share.

According to EverydayHealth.com, research published in the Institute of General Linguistics journal looked at several sets of twins and suggested that as many as 40% of them create their own language with each other as early as babyhood. Twin babies often seem to understand each other without speaking, and even as they grow and expand their vocabularies, they learn to communicate in unique ways through body language, special phrases, gestures, and more. You may as well not even try to understand it.

12 Saving One For Later

This interesting fact about twins shows just how awesome the world of medicine can be. Couples who undergo in-vitro fertilization procedures may be able to have twins who aren’t born at the same time, but rather, years apart!

There are a few couples who are able to share their stories of twins born in different years. One couple, Rachael and Richard, are pictured above. Rachael had her son, Oliver, via the procedure in 2014 after several failed attempts at conceiving naturally. The couple decided to have her other embryos frozen at the time. Two years later, brother Isaac was born using one of the frozen embryos. The boys, even though they’re two years apart, are technically twins born from the same batch of embryos! Even more amazing is that they almost shared the same birthdate, missing by just a few days.

11 You Are Not Alone

Recent studies are confirming that twin births are becoming more common. So, if you think you’re seeing more twin babies in the family than usual, you’re probably not imagining things! Research has suggested for a long time that there may be such a thing as a “twin gene,” meaning that twins may run in the family. But, genetics don’t have everything to do with the increase in twins.

Instead, scientists are relating the twin baby boom to the higher average age of mothers, which we’ve noted may lead to an increased chance of twins. And, the use of fertility treatments has also increased in recent years, which can up the odds as well. Who knows – in a few years we may all be seeing double even more than we are now!

10 Raise Your Left Hand

One of the most interesting facts about twins is one that few even know about, but plenty of research has suggested is real. If you know any twins who are left handed, there may be a scientific reason behind it. Being left handed is rare in the general population, with an estimated 10.8% of people preferring their left hand over their right. But, left handedness in twins is much more common, hovering around 17%. Usually, only one twin is left handed, but it’s not particularly uncommon for both to be left handed.

There’s no exact reason pinned, yet, for why left handedness is more common in twins. But, scientists used to theorize that anyone who is left handed must have had a twin in the womb that vanished. This came from the belief that one twin was left handed and one right handed. It’s an interesting theory, but it’s never been proven.

9 Bonded For Life

If you haven’t seen the viral video of these two gorgeous twins clinging to each other after they were born, then you’re missing out. Seriously – look it up! It’s one of the cutest things you’ll see. People who aren’t twins likely can’t imagine the amazing bond twins have. Parents see it with their twins, but no one other than twins knows just how it feels. The bond is a scientific fact backed by research.

One Italian study found that the twin bond begins in the womb, with several of the sets of twins in the study cradling each other as early as 14 weeks gestation. Even more interesting is the fact that the twins were incredibly gently with each other, especially when they reached toward sensitive parts like their eyes. Brother and sister bonding at its finest!

8 Head In The Clouds

If you’re tall, nearing your 40s, and have twins in your family – look out. You’re much likelier to have twins than others who don’t have these characteristics! Yes, even being tall may make you a likelier candidate to be a twin mommy. A 2006 study in the Journal of Reproductive Medicine showed that women who had twins and other multiple births were, on average, at least an inch taller than women who had singletons.

But why? There’s no solid conclusion, but the main researcher had his own theory. He believed that the cause may be insulin-like growth factor (IGF), a protein that responds to the body’s growth hormone. Increased levels of this protein may stimulate the follicles that release eggs, causing women to have an increased chance of ovulating two eggs.

7 In It For The Long Haul

If you have a set of twins, you (and they) may outlast your friends and family! A study from 2016 concluded that twins may live longer than the general population, and identical twins live even longer than fraternal twins. The research didn’t conclude a reason for the longer lifespan, but the theory of some experts is pretty amazing – and makes perfect sense. They believe that twins may have a constant, strong support system in their other twin, giving them the healthy mental support needed for a fulfilling life!’

But, it’s not only the twins that benefit. Another study concluded that moms of twins tend to live longer than the general population. This could be, experts say, because healthier women are usually able to carry healthy twins, so their better health can help them live longer.

6 Beyond Fraternal And Identical

Almost everyone has heard of the two “main types” of twins: identical and fraternal. Identical twins are from the same egg, essentially having the same DNA. Fraternal twins, however, come from different eggs that were both fertilized. But, there are actually more types of twins than you may realize.

One type is polar body twins. This happens when a cell forms into another egg while one egg develops. Usually, that cell, known as a polar body, doesn’t survive, but in rare cases it does, growing larger and forming another egg. The egg will have the same DNA as mom, but may be fertilized with different sperm from dad, creating an unusual, half-identical set of twins. Another type is known as mirror-image twins, where the twins do everything opposite, like favor opposite hands or part their hair on opposite sides. Some may even have birthmarks on opposite sides of their bodies, creating the “mirror-image” look.

5 A Twin May Disappear

Early ultrasounds can pick up a twin that may have disappeared early in your pregnancy. It’s not largely uncommon for vanishing twin syndrome to occur. This happens when one twin “vanishes”, usually during the first trimester, its tissues getting reabsorbed into mom’s body. Mom never knows it happens until the extra sac shows in the ultrasound without a baby inside. It’s estimated that this event occurs in as many as 30% of pregnancies with multiples.

Fortunately, it does no harm to mom or the other baby when it happens in the first trimester, although some moms may have slight symptoms similar to those of a miscarriage, like mild cramping or bleeding. However, if vanishing twin syndrome occurs later in the pregnancy, it could pose a risk to the other baby or mom, causing possible intrauterine growth restriction to the baby and potential preterm labor for mom.

4 Rainbow Of Colors

Twins are seriously an awesome thing to study, especially when you understand all the crazy genetics that happen behind the scenes. What happens when the parents of twins have two very different skin colors? Some amazing, beautiful twin colors!

It’s completely possible for twins to have very different skin colors, just like their parents. After all, mixed-race siblings who aren’t twins can fall on different ends of the spectrum, too. So, twins may also inherit different shades. A set of twins born in 2016 – Kalani and Jarani – went viral with their gorgeous, but very different, skin tones. Jarani has dark skin and eyes like her daddy and Kalani has light skin and blue eyes like her mommy. And, yes, they can even be identical! The couple above had identical twin girls with very different skin tones, thanks to the many genes that control skin color.

3 Who's Your Daddy?

One of the most common questions about twins is, “Can they have different dads?” although many that hear the answer (yes!) still don’t believe it’s possible. But, it is, and plenty of research has confirmed that some fraternal twins can have different dads. If you’re not understanding how that can work, here’s a brief explanation:

Fraternal twins happen when two eggs get fertilized (unlike identical twins, in which two babies come from one egg splitting). It’s entirely possible that, if a woman is intimate with two different men in a short timeframe and released two eggs during ovulation, both of them could have been fertilized by the different men, resulting in twins with different dads. The process even has a name: heteropaternal superfecundation. A similar process can even happen (although, rarely) if sperm accidentally gets mixed up during an IVF procedure.

2 Competing For Success

If you find that your twins have an amazing bond yet know exactly what to do to get on each other’s nerves – and one-up each other at any given moment – just know you’re not alone. It’s extremely common for twins to be very competitive with each other, which may drive parents bonkers, but may actually be a good thing for them.

Just as any pair of siblings, each of your twins has unique strengths and weaknesses. It’s good for them to balance each other out and some healthy competition can motivate them to keep pursuing goals. When they’re young and reaching milestones all the time, especially, one twin’s persistence can motivate the other to reach a milestone he may be lagging a bit with. If they’re not harming each other, consider the competition a healthy form of bonding and fostering their individuality – in their unique way, of course.

1 Who Are You, Again?

Probably one of the scariest things for parents who find out they’re having twins is wondering if there will ever be a time that they mix up their babies. To others, it may not seem like a big deal to mix up their names when they’re younger, but in reality, it’s more than just a name – it’s an identity. Until your babies start developing unique personalities, it can be quite easy to mix them up if they look like little replicas of each other.

But, according to twin parents, it’s more common than you think, so don’t feel like a terrible parent if it happens to you. One woman answered a question on Quora about the subject. She’s a twin herself and said that her parents have mixed her and her sister up throughout her lifetime, and even in adulthood. Most agree that it happens more often when parents are overtired than when they’re awake and alert. So, blame it on the Mom Brain!

Sources: BellyBelly.com.au, DailyMail.co.uk, EverydayHealth.com, FoxNews.com, Quora.com, WebMD.com, WhatToExpect.com