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Why 1 in 5 Pregnancies End in Miscarriage

Causes of spontaneous abortion

Why so many fertilized eggs don't thrive

It’s thought that roughly 40 percent of all fertilized eggs are lost to spontaneous miscarriages before women even know they are pregnant. Once a woman does know she’s pregnant, still up to one in five pregnancies end in miscarriage — usually in the first trimester.

This week in the journal Current Biology, scientists report they now how a strong idea as to why so many pregnancies go wrong.

Here’s what they found:

In an ideal world, when a woman’s eggs mature and divide before ovulation, they do so evenly. First the chromosomes (23 pairs of them) line up, then wait for a cue (called the “spindle assembly checkpoint”), then they divide into two daughter cells, each with 23 chromosomes — ready to match up with a sperm’s 23.

But when researchers watched mice egg cells divide (in a similar process to humans), they found that a lot of the egg cells jump the gun — sending the signal to divide before all the chromosomes have lined up. That makes for too few chromosomes in one egg cell and too many in another. When one of those egg cells matches up with sperm (which seem to more reliably have 23 chromosomes), the resulting fertilized egg has the wrong total number of chromosomes.

Either way, most babies don’t make it past an early stage of development if they have extra or missing chromosomes. The exception, of course, is Down’s Syndrome — three chromosome 21′s.

The March of Dimes says that about 50 percent of miscarriages are caused by chromosomal abnormalities. This study suggests that many of those early miscarriages are the result of an egg’s tendency for too-quick division. Of course there’s no way (or reason) to intervene here for the natural conception process, but the scientists say this understanding could help sort viable eggs during IVF.

Image: flickr

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