Prenatal Screening Tests for the Detection of:
Down Syndrome, Trisomy 18 and Open Neural Tube Defects

Advances in prenatal screening have resulted in new tests that offer an improved detection rate and fewer false positives in the detection of chromosome abnormalities. These include nuchal translucency (NT) ultrasound, and new biochemical markers (PAPP-A and DIA). Timing of these tests beginning at 11 weeks’ gestation requires discussion early in pregnancy.

What disorders are being screened for?

Prenatal screening gives a woman her individual risk of having a child with Down syndrome, trisomy 18 and open neural tube defects. It does not screen for all chromosome abnormalities, so some may be missed. Following positive results, women will need to decide whether to go on to have diagnostic testing (i.e. CVS or amniocentesis). Prenatal screening should be offered as part of a program where diagnostic testing, counselling and follow up are available.

Down Syndrome (trisomy 21):

Down Syndrome characteristics include: intellectual disability of varying severity, characteristic facial appearance, hypotonia & other less common congenital anomalies. The general population incidence of Down syndrome is about 1 in 1000, but varies with maternal age.

Prenatal ultrasound findings: congenital heart defects (40%), intestinal obstruction (12%). Approximately 1/3 of affected fetuses will have normal ultrasounds at 18-20 weeks.

Trisomy 18 (Edwards syndrome):

95% of pregnancies with trisomy 18 will result in a miscarriage or stillbirth, and 95% of these liveborn infants die by 1 year. Surviving infants will have severe intellectual disabilities and multiple congenital anomalies. The general population incidence of trisomy 18 is 1 in 6,000, but varies with maternal age.

Prenatal ultrasound findings: congenital heart defects (90%), choroid plexus cysts, distinct hand posture, club feet, micrognathia, intrauterine growth retardation and others. Though rare, affected fetuses may have a normal ultrasound at 18-20 weeks.

Open Neural Tube Defects (NTD) - including anencephaly and spina bifida:

Anencephaly is lethal. Most babies with spina bifida survive and may have problems ranging from hydrocephalus, paralysis and learning/intellectual disabilities to no physical or mental disabilities. Non-gestational diabetes mellitus, anticonvulsant medications, family history of NTD and hyperthermia result in a higher chance of an affected child. The general population incidence in North America is about 1 in 2,000, and does not vary with maternal age.

Counselling Tip:
“A screening test can tell us if your baby has a higher than average chance of having a certain disorder. It does not tell us if your baby truly has the disorder or not. With screening, most babies with Down syndrome will be detected, but some will be missed.”

Things to keep in mind:

• Informed choice - Before ordering the test, discuss benefits, risks and limitations.
• Autonomy - The patient should choose whether to have prenatal screening.
• What prenatal screening options are available in your area?
• What option is most suitable for your patient?
• Which test will provide the optimal care for your patient?
• A screening test is not diagnostic.

Call the ministry INFOline at 1-866-532-3161
TTY: 1-800-387-5559

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