What is a miscarriage?
Miscarriage is the loss of a pregnancy in the first 20 weeks. (In medical articles, you may see the term "spontaneous abortion" used in place of miscarriage.) About 10 to 20 percent of known pregnancies end in miscarriage, and more than 80 percent of these losses happen before 12 weeks.
This doesn't include situations in which you lose a fertilized egg before a pregnancy becomes established. Studies have found that 30 to 50 percent of fertilized eggs are lost before or during the process of implantation – often so early that a woman goes on to get her period at about the expected time.
What causes a miscarriage?
Between 50 and 70 percent of first-trimester miscarriages are thought to be random events caused by chromosomal abnormalities in the fertilized egg. Most often, this means that the egg or sperm had the wrong number of chromosomes, and as a result, the fertilized egg can't develop normally.
Sometimes a miscarriage is caused by problems that occur during the delicate process of early development. This would include an egg that doesn't implant properly in the uterus or an embryo with structural defects that prevent it from developing.
When the fertilized egg has chromosomal problems, you may end up with what's sometimes called a blighted ovum (now usually referred to in medical circles as an early pregnancy failure). In this case, the fertilized egg implants in the uterus and the placenta and gestational sac begin to develop, but the resulting embryo either stops developing very early or doesn't form at all.
Because the placenta begins to secrete hormones, you'll get a positive pregnancy test and may have early pregnancy symptoms, but an ultrasound will show an empty gestational sac. In other cases, the embryo does develop for a little while but has abnormalities that make survival impossible, and development stops before the heart starts beating.
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Originally Posted by Chi
