Screening for Down Syndrome

Prenatal Screening and Diagnosis:

Down syndrome is often associated with pregnancies at an older age, so it's advisable for older women to undergo amniocentesis screening. Amniocentesis, typically performed around 14-16 weeks of pregnancy, is the most common and reliable invasive diagnostic test. It's worth noting that the procedure carries a slight risk of pregnancy loss (approximately 1 in 200-300).

Other invasive diagnostic tests include chorion villi biopsy (CVS) in the first trimester and cordocentesis, which involves collecting fetal blood from the umbilical vein using an ultrasound-guided needle. For quicker results, fluorescence in situ hybridization (FISH) analysis can be performed on both uncultured cells (interphase cells) and cultured cells (metaphase spreads). However, these results should be confirmed through chromosome analysis from cultured fetal cells.

Additional screening tests involve assessing maternal serum alpha-fetoprotein (MSAFP) levels, human chorionic gonadotropin (hCG) levels, and unconjugated estriol (uE3) levels.

Down syndrome can also be suspected based on prenatal ultrasound examinations during routine check-ups or for women at a higher risk. Prenatal ultrasound markers include:

1. Nuchal (neck) fold thickening, which identifies about 75% of Down syndrome cases.
2. Shortened humerus or femur (leg bones) length, detecting approximately 31% of cases.
3. Cystic hygroma (a cystic structure in the neck region).
4. Duodenal atresia or stenosis (indicated by the "double bubble sign").
5. Cardiac defects, with endocardial cushion defect and atrial and ventricular septal defects being the most common.
6. Echogenic bowel (where the bowel reflects sound waves).
7. Renal pyelectasis (dilatation of the kidney pelvis).

Risk for Down Syndrome:

Advanced maternal age is the primary well-documented risk factor for Down syndrome, with the following risk statistics:

1. At a maternal age of 35 years, the risk is 1 in 385.
2. At a maternal age of 40 years, the risk is 1 in 106.
3. At a maternal age of 45 years, the risk is 1 in 30.

Couples who have had one child with the most common type of Down syndrome face a slightly increased risk (around 1%) of having another affected child. A parent who is a carrier with a translocation has a significantly higher risk of having a baby with Down syndrome, dependent on the type of translocation. The recurrence risk can be as high as 100% if the parent carries a 21q21q translocation or an isochromosome, a type of abnormal chromosome with two identical long arms of the 21st chromosome due to the duplication of the long arm and the loss of the short arm. Therefore, prenatal screening and genetic counseling are crucial. Consult your doctor for guidance.

People with Down syndrome rarely have children. Approximately 15-30% of women with trisomy 21 are fertile, and they have a 50% risk of having an affected child. There is no evidence to suggest that an affected man can father a child.