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What is a miscarriage?
A miscarriage occurs when a recognized pregnancy
fails to progress, resulting in loss of the
pregnancy. This can include spontaneous pregnancy losses,
or missed abortions, in which the loss of a viable pregnancy is discovered
some time after the fetus stopped growing. Pregnancy losses are very common,
with up to of 15% of recognized pregnancies resulting in a miscarriage,
usually in the first trimester. Many couples are very interested in seeking
an explanation for a pregnancy loss.
What are the common causes of miscarriages?
Common genetic causes of reproductive losses include:
- chromosomal abnormalities in the fetus,
including aneuploidy
- chromosomal imbalance in the fetus due
to a parent with a balanced chromosomal translocation
- genetic syndromes in the fetus
- a predisposition towards blood clotting
in the mother (thrombophilia)
- Testing is available
to study some of the common causes of pregnancy
loss.
What are common chromosome abnormalities?
Chromosomal abnormalities account for a significant proportion of fetal wastage,
with approximately 60% of spontaneous abortions, 7% of stillbirths, and
1% of neonatal deaths being caused by a chromosome abnormality. Chromosomes
are the actual pieces of inherited material passed from both parents to
the child. If too many or too few chromsosomes are passed down to the
child, the pregnancy will typically be lost as a miscarriage. A chromosome
analysis is required to identify any chromosome abnormality as the cause
of a miscarriage or stillbirth. When too many or too few chromosomes are
passed down to the child, there is usually no history of these problems
in the family, and there is not a high risk of a chromosome abnormality
existing in the parents.
Are there some chromosome abnormalities that
are inherited?
Yes. Couples who have a history of repeated pregnancy loss are at an increased
risk of carrying an additional chromosomal abnormality: a chromosomal translocation. A
carrier of a balanced translocation is completely normal, however may contribute
unbalanced products to a pregnancy, resulting in repeated miscarriages or
infertility.
Chromosome analysis in couples experiencing
two or more reproductive losses show that for
about 6% of couples, one parent is a carrier
of a chromosome translocation. Identifying
a translocation carrier helps 1) determine
the likely cause of the previous losses, 2)
identifies the carrier's risk of having a reproductive
loss; a live born unaffected or an affected
baby with future pregnancies, and 3) identify
other members of the family who may also be
carriers of the translocation.
What is Thrombophilia?
Thrombophilia refers to the excessive tendency to clot blood. During a pregnancy,
this can result in an interrupted blood supply to the growing fetus, and
result in a pregnancy loss or a poor pregnancy outcome. Most women do not
know that they may have this tendency prior to experiencing a pregnancy loss,
thus screening of all women with multiple pregnancy losses is indicated. Some
of the reasons for thrombophilia can be due to either ongoing or temporary
imbalances in an immune status, inherited deficiencies of proteins used to
maintain the balance between bleeding and blood clotting, or deficiencies
of proteins which decrease the tendency to clot. A large panel of tests
are frequently run, depending on the exact problems being experienced and
other factor in a woman's family history.
Who should consider genetic testing for pregnancy
losses?
Complete genetic services are provided for couples who experience reproductive
losses. Most couples who have experiences two or more miscarriages are at
increased risk of either partner being a carrier of a balanced chromosome
rearrangement. In addition, these additional tests are typically ordered.
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