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    A User's Guide to Prenatal Tests

    First Trimester Tests

    Here are some tests you may undergo during the first trimester of your pregnancy:

    Blood tests: During one of your initial exams, your doctor or midwife will identify your blood type and Rh factor, measure the level of iron in your blood, check for immunity to rubella (German measles), and test for hepatitis B, syphilis and HIV. Depending on racial, ethnic or family background, you may be offered tests and genetic counseling to assess risks for diseases such as Tay-Sachs, canavan, cystic fibrosis, thalassemia and sickle-cell anemia (if these weren't done at a preconception visit).

    Urine tests: You'll also be asked early on for a urine sample so that your doctor or midwife can look for signs of kidney infection and, if necessary, to confirm your pregnancy by measuring the level of hCG, a hormone secreted by the placenta. (A blood test to confirm pregnancy may be used instead.) Urine samples will then be collected regularly to detect glucose (a sign of diabetes) and albumin (a protein that indicates pre-eclampsia, which is pregnancy-induced high blood pressure).

    Cervical swabs: You'll also have a Pap smear to check for cervical cancer, and vaginal swabs may be taken to test for chlamydia, gonorrhea and bacterial vaginosis, an infection that can cause preterm birth. Treatment of these conditions will prevent complications for your newborn.

    Chorionic villi sampling (CVS): If you're 35 or older or have a family history of certain diseases, you'll be offered this optional, invasive test between 10 and 12 weeks, which can detect many genetic defects, such as Down syndrome, sickle-cell, cystic fibrosis, hemophilia, Huntington's chorea and muscular dystrophy. The procedure involves either threading a tiny catheter through your cervix or inserting a needle into your abdomen to obtain a tissue sample from the placenta. The procedure carries a 1% risk of inducing miscarriage and is about 99% accurate in ruling out certain chromosomal birth defects. But in contrast to amniocentesis, it does not help in detecting open spine disorders, such as spina bifida and anencephaly, or abdominal wall defects.

    A promising new procedure that combines a blood test with an ultrasound to screen for Down syndrome may be available between 10 and 14 weeks. The results of a blood test that measures either hCG and PAP-A (pregnancy-associated protein A) in maternal blood are used with an ultrasound measurement of the fetus' skin at the back of its neck (called nuchal-translucency). The procedure may be able to pick up a substantial portion of Down syndrome cases and other genetic diseases, says Dr. John Larsen, professor of obstetrics, gynecology and genetics at George Washington University. However, as with all screening methods, a more invasive diagnostic technique like CVS is used to follow up a positive screen.

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