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Miscarriage is the most common type of pregnancy loss, according to the American College of Obstetricians and Gynecologists. Anywhere from 10 to 25 percent of all known pregnancies end this way, and more than 80 percent of these miscarriages occur within the first 12 weeks. There is a lot of debate over the actual rate of miscarriage, however, because you may have one before you even know you're pregnant. In these cases, the loss usually goes undocumented. When these unrecognized losses are taken into consideration, the estimated rate of miscarriage is anywhere from 50 to 75 percent.

Causes of Miscarriage

A miscarriage (technically called a spontaneous abortion) is a pregnancy that ends on its own within the first 20 weeks of gestation, and occurs when the fetus and placenta separate from the uterine wall. While the causes of miscarriage are not well understood, there are several factors believed to contribute to their occurrence.

Chromosomal abnormalities

The most common reason for the loss of a pregnancy during the first trimester is chromosomal abnormality, which means something is wrong with the chromosomes of the embryo. Chromosomes are the tiny structures in each of our cells that carry our genes, which dictate everything from the color of our eyes to the functioning of our internal organs. We all have 23 pairs of chromosomes, or 46 total, with one chromosome per pair coming from our mothers and one from our fathers. Defects in chromosomes are found in more than half of miscarriages occurring in the first 13 weeks.

Chromosomal abnormalities occur before pregnancy when immature egg and sperm cells divide unevenly. Rather than form with 23 chromosomes, the mature egg or sperm cells end up with too many or too few. With the wrong number of chromosomes, the embryo has an abnormality and is usually miscarried. Miscarriages are said to eliminate about 95 percent of fertilized eggs or embryos with genetic problems, which could be nature's way of ending a pregnancy in which the child would have been unable to survive. Most chromosomal problems happen by chance, have nothing to do with you or your partner, and are unlikely to recur.

Below are the most common chromosomal defects.

  • Trisomy - Trisomy is a condition that occurs when instead of 23 matched pairs of chromosomes, there are 22 pairs plus one set of three. Most fetuses with trisomy are miscarried during the first trimester. Of those who do survive, most have Down syndrome, also called trisomy 21 because they have a third chromosome 21. Others who survive with trisomy may have trisomy 13 or 18, which are almost always associated with profound mental retardation and other serious malformations at birth. These babies rarely live more than a few days or months.

  • Monosomy - Monosomy is having a single copy of a particular chromosome, rather than the usual pair. Sometimes when an egg or sperm cell is forming, an error will cause it to have a missing sex chromosome. Only 1 percent of these pregnancies will survive. Female babies who are born with only one X chromosome have a condition called Turner syndrome. This condition, also known as monosomy X, occurs in one in every 2,500 female newborns, causing them to be shorter than their peers and to not mature sexually as they become adults.

  • Polyploidy - Polyploidy occurs when cells contain more than two copies of each of their chromosomes. This may be a result of an egg being fertilized with more than one sperm. Additional chromosomes can cause multiple anomalies, such as defects of the heart and central nervous system. There is no chance for long-term survival, and most of these cases end in miscarriage.

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