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Recurrent Pregnancy Loss Part 2

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A blog by Joseph A. Hill, III, MD, Fertility Centers of New England

Chromosome disorders are the most common cause of miscarriage. Approximately 60 percent to 70 percent of pregnancy losses — both single and recurrent (two or more) miscarriages — are chromosomally abnormal.

The Most Common Abnormalities

The most common chromosomal abnormality is trisomy 16, which occurs in 26 percent of losses. Trisomy 16 is incompatible with life, and the mother miscarries in the first trimester, within 11 weeks of gestation.

The second most common chromosome abnormality in losses is monosomy X (45,X) -- approximately 90 percent of women who conceive a pregnancy with this abnormality have a miscarriage. Ten percent may continue the pregnancy to term delivery, resulting in a little girl born with Turner’s Syndrome, a condition with characteristic physical abnormalities such as short stature, swelling, broad chest, low hairline, low-set ears and webbed necks, as well as non-working ovaries and sterility.

The third most common chromosome abnormality in miscarriages is trisomy 22 (three copies of chromosome 22). Progression to the second trimester or term birth are rare.

Trisomy 21 is the fourth most common chromosomal abnormality leading to miscarriage. It is estimated that approximately 80 percent of trisomy 21 pregnancies end in a miscarriage (pregnancy loss before 20 weeks of gestation) or intrauterine fetal demise (pregnancy loss after 20 weeks of gestation), while 20 percent may progress to term delivery. Babies born with trisomy 21 have Down syndrome, which can range from mild to severe.

Not the Parents' Fault

You may be surprised to find out that only 3 percent of individuals having three or more pregnancy losses have an inborn chromosome abnormality themselves. But more than 60 percent of pregnancy losses are due to chromosomal defects. Why?

There are many factors that come into play when an egg and sperm unite and form that first cell — factors that are not within a person's control. Most chromosomal abnormalities in early pregnancy losses arise spontaneously because of non-disjunction (the failure of chromosome pairs to separate properly during cell division) or because of other genetic errors in the egg, the sperm or in the early embryo.

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