There's more to the formation of twins than you might have thought. On today's Moment of Science, we discuss a problematic type of embryonic development.

As you may already know, there are different kinds of twins: fraternal and identical. Fraternal is when two different eggs are fertilized by two separate sperm. Even though fraternal twins can look very much alike, on average they only share fifty percent of their genes, the same as any other siblings. For identical twins, the process is much different. As the egg is fertilized, the embryo splits, creating two babies with the same genes.

But did you know that there are different kinds of pregnancies involving identical twins, depending on when the embryo splits? If it splits within two or three days after the egg has been fertilized, the twins grow in their own separate amniotic sacs. But if it splits a week or so later, the babies develop in the same sac with nothing to seperate them. They're called mono amniotic twins.

The problem is that mono amniotic twins almost always tangle their umbilical cords. If the cords twist too tightly, they can become compressed, cutting off nutrients to the twins. It's not uncommon for one or both babies to be still born or develop chronic health problems. Luckily, monoamniotic twins are pretty rare. But if they do occur, the best thing is to consult with a doctor and weigh the options.