About one in 30 babies born in the U.S. is a twin. Perhaps because of their rarity, people are fascinated by them.

Twins are obviously two distinct people, yet they can share so many characteristics that it’s hard to remember that at times. Here are four things you may not have known about twins.

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1. Twins don’t have the same fingerprints or DNA.

While identical twins start with the same DNA at conception, they quickly acquire hundreds of genetic changes through development. That means that by the time they are born, identical twins no longer have identical DNA.

The same is true of their fingerprints. When the embryos start out, they have the same fingerprints due to having the same DNA at the time. However, as the embryos reach their sixth week of development, they begin to move. By touching the amniotic sac, they form new contours on their fingerprints, giving them each a unique set.

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2. Some identical twins are known as mirror-image twins.

About a quarter of all identical twins have mirror image features. This means instead of each having a birthmark on the right side of their faces, one will have it on the left side while the other has it on the right.

Examples of features that can show up as mirror images in twins are birthmarks, freckles, moles, and dimples. Cowlicks (the whorls in your hair) will even swirl in opposite directions in mirror-image twins.

In some extreme cases, internal organs can even be reversed. The exact cause of mirror twinning is unknown. Some theorize that it occurs when an egg splits later that usual but not quite late enough to cause the embryos to become conjoined.

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3. Raising twins can actually be easier than raising two children spaced farther apart.

We all know the trope of the soon-to-be parents who find out they’re having twins. Their eyes widen and they fan themselves to prevent fainting.

While having twins is certainly more expensive and requires more work than having a single child, it can be cheaper than having two children spaced out by a year or more. Since they go through phases and stages together, meal-making, doctor’s appointments, and other tasks can be easier to carry out.

As the twins get older, they’ll likely do similar activities. This makes scheduling and carpooling simpler. Best of all, when you have twins, your children have a ready-made best friend to help occupy their time.

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4. It’s not true that having twins skips a generation.

Obviously, this phenomenon can skip a generation; it’s just that it’s not statistically more likely to do so. In fact, having identical twins is not related to genetics at all.

On the other hand, having fraternal twins is largely determined by genetics. A woman who has a fraternal twin or who has fraternal twins as siblings is two and a half times more likely to give birth to fraternal twins. 

There is a whole host of other strange factors that increase the odds of having twins. Most people know that older women and those who use in vitro fertilization are more likely to have twins.

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Taller women and those who eat more dairy are also more prone to having twins. This is due to having higher levels of IGF, a hormone that can increase sensitivity in ovaries. 

Having twins or being twins is special. If you or someone you love is a twin, you should feel lucky that you get to experience this one-of-a-kind phenomenon. It’s one of those little things that makes life infinitely more interesting.