Special fetal tests detect in utero fetal defects
Depending on a mother's risk profile, the following special fetal tests may be ordered to detect early fetal birth defects.
Ultrasound utilizes sound waves that bounce off tissues. These waves of sound are deflected and converted into images of the fetus, which then appear on a screen.
Ultrasound detects the number of fetuses, age and growth rate of the fetus, location of the placenta, fetal position and heart rate, amount of amniotic fluid in the uterus and some birth defects.
Alpha-Fetoprotein (AFP) Screen
It is performed between weeks 15-18. Every fetus produces alpha-fetoprotein that passes from the amniotic fluid into the mother's blood.
If a fetus has an open neural tube defect (spina bifida), excessive quantities of alpha-fetoprotein leak into the mother's blood. Low levels of alpha-fetoprotein are associated with Down syndrome. This test identifies 90 percent of open neural tube defects and 25 percent of Down's cases. False positives occur almost 5 percent of the time.
With amniocentesis, fetal cells present in the amniotic fluid are collected from a needle inserted into the abdomen. These cells are then cultured and grown in order to detect chromosomal abnormalities, including Down syndrome. The test is performed between 14 - 18 weeks.
Chorionic Villus Sampling
Chorionic villi are microscopic projections that compose the placenta. They have the same genetic constitution as the fetus. The villi sample is collected either through a needle in the abdomen or through a vaginal catheter and cultured to detect chromosomal abnormalities. This test is performed in the first trimester.
Various versions of this blood test exist as double test or triple test. They are all types of serum screening and differ only slightly from one another. The serum screening test is a blood test offered between 15-20 weeks. It combines the results of AFP test with measurement of other chemicals in your blood to give a risk factor for your baby to be born with Down's syndrome.
As the AFP test results are combined with this, the risk factor works out for spina bifida as well.
Cordocentesis is the extraction of a blood sample from the umbilical cord. This test can be carried out late in pregnancy, between 18 to 24 weeks. It is carried out in women who are at a higher risk of chromosomal/genetic abnormalities.
A very modern and fairly new procedure, fetoscopy involves extraction of umbilical cord and placental samples to check for certain skin and blood disorders that cannot be detected by amniocentesis. This can be carried out after 16 weeks of pregnancy and carries a 3-5% risk of miscarriage.
A miscarriage is the loss of a baby before the 20th week of pregnancy. Many losses occur before a woman knows she is pregnant. About 15 percent of recognized pregnancies and as many as 50 percent of all pregnancies end in miscarriage.
Most miscarriages occur in the first trimester (12 weeks) of pregnancy. The causes of miscarriage are not entirely known. The most common reason for first-trimester losses is a chromosomal abnormality in the baby. Infections, hormonal problems and health problems in the mother can also cause a miscarriage.
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