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Pregnancy Weeks 13-18 - Page 8

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Healthy Pregnancy Newsletter - Checking It Out

Your baby

What is an amniocentesis?

An amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus to diagnose chromosomal disorders and open neural tube defects (ONTDs) such as spina bifida. Testing is available for other genetic defects and disorders depending on the family history and availability of laboratory testing at the time of the procedure.

An amniocentesis is generally offered to women between the 15th and 20th weeks of pregnancy who are at increased risk for chromosome abnormalities, such as women who are over 35 years of age at delivery, or those who have had an abnormal maternal serum screening test, indicating an increased risk for a chromosomal abnormality or neural tube defect.

In addition to your regular prenatal visits for weight gain, blood pressure, and pregnancy wellness, you may be offered a blood test to screen for specific problems that may occur in an unborn baby. The most common test is an alpha-fetoprotein screening (AFP), which measures the level of a protein normally produced by the unborn baby's liver. The protein is present in the amniotic fluid surrounding the fetus and crosses the placenta into the mother's blood. The AFP blood test is also called MSAFP (maternal serum AFP) and is generally performed between 16 and 18 weeks of pregnancy.

AFP levels that are too high or too low may signal a problem such as the following:

  • Open neural tube defects (ONTD) such as spina bifida
  • Down syndrome
  • Other chromosomal abnormalities
  • Defects in the abdominal wall of the fetus
  • Twins. More than one fetus is making the protein
  • A miscalculated due date, as the levels vary throughout pregnancy

AFP screening may be included as one part of a 2-, 3-, or 4-part screening, often called a multiple marker screen. If the results of AFP or other markers are abnormal, additional testing may be needed. Usually an ultrasound is performed to confirm the dates of the pregnancy, and to look at the unborn baby's spine and additional parts of the body for possible defects. An amniocentesis may be needed for accurate diagnosis.

It's important to remember that multiple marker screening is not diagnostic. This means it is not 100 percent accurate, and is only a screening test to determine if you should be offered additional testing. There can be false-positive results (indicating a problem when the fetus is actually healthy) or false-negative results (indicating a normal result when the fetus actually does have a health problem). Talk with your health care provider to determine whether MSAFP testing is appropriate for your pregnancy.

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