The country's latest set of rare identical triplets headed home Monday after a month in neonatal intensive care at Houston's Memorial Hermann Southeast Hospital. Addison, Kinsley and Savannah Harris were born on Dec. 1, each clocking in between three and three and a half pounds.

"Hopefully they don't get mixed up," their mother Stephanie Harris joked to NBC.

It won't surprise you to learn that identical triplets are rare: Doctors frequently call these births a one in a million occurrence. They're actually more like a 20 or 30 in a million occurrence, according to research, but that's still pretty nifty.

So how do three babies end up with the same DNA? The same way two babies do. Fraternal or dizygotic twins (which occur more than twice as often as identical or monozygotic twins) are really just siblings that share a gestation. These multiples occur when more than one egg is fertilized and successfully grows into an embryo during the same pregnancy. In 2014, twin birth rates hit an all-time high in the United States, mostly because couples that use fertility drugs or in-vitro fertilization are likely to have fraternal multiples.

But monozygotic multiples come from a very different phenomenon: A single egg is fertilized by a single sperm, but that fertilized egg (called a zygote) then splits up, ultimately forming multiple, identical embryos that continue to grow independently. This can actually occur for over a week after the initial fertilization, and no one is really sure why it happens. Contrary to popular belief, identical twins (or triplets or quadruplets) don't actually run in families. Fraternal multiples might, if the women in a particular family share some genetic trait that makes them more likely to release two or more eggs during an ovulation cycle instead of one. But for now, there's no evidence of a genetic predisposition for zygote splitting.

Doctors can often spot the difference between monozygotic and dizygotic multiples without a DNA test, because identical twins usually share a placenta. This isn't always the case, especially when the split occurs late in the game, and a recent study found that some parents told they're having fraternal twins may actually get a matched set. But the opposite scenario – where fraternal multiples end up sharing a placenta instead of each getting their own – is much less common, so that's a tougher mistake to make.

In the case of the Harris triplets, early ultrasounds revealed two fetuses sharing a sac. The third fetus – sharing the same placenta – wasn't spotted until later. When an even more rare set of identical quadruplets was born in 2014, their parents thought they were having triplets until the delivery was almost over. The odds of that event? A staggering 1 in 13 million.

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