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Screening for Down Syndrome Prenatal screening and diagnosis

Because Down syndrome is associated with getting pregnant  at an older age, older women should generally have screening with amniocentesis. Amniocentesis (routinely performed at 14-16 weeks' gestation) is the most commonly used and most reliable invasive diagnostic test. The procedure is associated with a small risk of pregnancy loss (1 in 200-300). Other invasive diagnostic tests include chorion villi biopsy (CVS) in the first trimester and cordocentesis (collection of fetal blood  from the umbilical vein with an ultrasound-guided needle). Fluorescence in situ hybridization (FISH) analysis may be performed to analyze interphase cells (uncultured cells) and metaphase spreads (cultured cells) for speedy results. However, these results should be confirmed with chromosome analysis from cultured fetal cells.

Other screening tests include testing for low maternal serum alpha-fetoprotein (MSAFP), high human chorionic gonadotropin (hCG), and low unconjugated estriol (uE3).

Down syndrome may also be suspected based on prenatal ultrasonography in routine examination or in women at high risk. The prenatal ultrasound markers include the following:

    A.   Nuchal (neck) fold thickening - Identifies 75% of Down syndrome fetuses

    B.   Shortened humerus or femur (leg bones) length - Detect about 31% of cases

    C.   Cystic hygroma (cystic structure in neck region)

    D.   Duodenal atresia or stenosis (double bubble sign)

    E.   Cardiac defects - The most common defects are endocardial cushion defect with atrial and ventricular septal defects.

    F.   Echogenic bowel (the bowel reflects sound waves)

    G.   Renal pyelectasis (dilatation of the pelvis of the kidney)


Risk for Down syndrome

Advanced maternal age remains the only well-documented risk factor for Down syndrome.

    i.    With a maternal age of 35 years, risk is 1 in 385.

    ii.    With a maternal age of 40 years, risk is 1 in 106.

    iii.    With a maternal age of 45 years, risk is 1 in 30.

Couples who have had one child with the most common type of Down syndrome are at a slightly increased risk (about 1%) for having another affected child. A carrier parent with a translocation has much higher risk of having a baby with Down syndrome, depending on the type of translocation. The recurrence risk is as high as 100% if the carrier parent has 21q21q translocation (between the two 21st chromosomes) or isochromosome (an abnormal chromosome consisting of two identical long arms of the 21st chromosome due to duplication of the long arm and the loss of the short arm). So prenatal screening and genetic counseling are important. Talk to your doctor.

People with Down syndrome rarely reproduce. From 15-30% of women with trisomy 21 are fertile, and they have a 50% risk of having an affected child. No evidence exists of an affected man fathering a child.


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