A Moment of Science

Monoamniotic Twins

Did you know that there are other kinds of twins besides fraternal and identical?

identical twin baby girls

Photo: Ruth L (flickr)

Identical twins occur when after the egg is fertilized and becomes a very young embryo it splits, creating two babies with the same genes.

Did you know that there are other kinds of twins besides 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 50% of their genes, the same as any other siblings.

Identical twins occur when after the egg is fertilized and becomes a very young embryo it splits, creating two babies with the same genes.

However, 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. However, if it splits a week or so later, the babies develop in the same sac with nothing separating them. They’re called monoamniotic twins.

The problem is that monoamniotic 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.

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