Miscarriages in the first trimester of pregnancy
What is a chromosome disorder and what is its significance?
Miscarriages in the first trimester of pregnancy are relatively common, and occur
in approximately 20-25% of pregnancies. The causes of miscarriages are multiple,
and cover a range of factors associated both with the fetus and the mother. Fetal
causes include chromosome disorders and syndromes, non-chromosomal genetic disorders,
and abnormal fetal development. Maternal factors include chronic illnesses, viral
infections during pregnancy, abnormalities of the uterus, etc. There are also many
factors that are as yet unknown.
Abnormal fetal chromosomal structure is one of the most common causes of miscarriage.
Usually this is a new occurrence arising in the fetus, but in a small number of
cases it is the result of a chromosomal problem in one of the parents. Here the
parent carries a structural change in one of his/her chromosomes in a balanced manner
or inversion). The parent
is completely healthy, because there is no change in the total amount of genetic
material, but the fetus may receive an unbalanced chromosomal complement (insertions
or deletions of complete chromosome segments or whole chromosomes). In some cases,
the change causes a miscarriage.
Because of this possibility, in cases of multiple miscarriages, sometimes a sample
of the fetus removed by curettage is sent to a genetic institute for examination
of the fetal chromosomal structure (karyotype).
This is performed only in cases of multiple miscarriages (more than 3).
In some cases the parents' karyotypes are also analyzed. About 1% of women have
3 or more consecutive miscarriages and about 10% have 2 or more.
In cases of women who have recurrent miscarriages (three or more, although many
physicians consider two to be the borderline), it is important for both partners
to undergo blood tests for chromosome analysis in order to determine whether either
carries a balanced structural change (translocation or another problem, such as
mosaicism, marker, etc.)
According to reports in the literature, such changes are not rare, and in 5% of
cases of recurrent miscarriage, a
balanced chromosome problem in a parent is identified. Such couples have
a higher risk of having a fetus with an
unbalanced chromosome disorder in each additional pregnancy, and prenatal
diagnosis should be offered in each pregnancy in order to prevent the birth of an
infant with a chromosome disorder leading to defects or mental retardation.
It is important to consult one's physician in all cases of recurrent miscarriage.
A history of miscarriages in other family members and/or cases of intrauterine death
and/or the birth of an infant with defects is important, as the presence of these
can mean an increased risk for the couple concerned.
In addition, tests for excessive clotting tendency (blood clotting) should be carried
out in cases of recurrent miscarriage, especially if there is a history of intrauterine
death or late pregnancy loss. It is possible to test the mother for a genetic predisposition
for an increased blood clotting tendency, and if this is found, an anti-clotting
drug such as aspirin, etc., is administered.