The BC Prenatal Genetic Screening Program

by Susan Miller, BScN

Expectant parents want to be assured that their unborn baby will be healthy. One way to find out prenatally about baby’s well-being is through prenatal genetic screening. Free genetic screening is available to every pregnant woman in B.C. In February, new guidelines were introduced that provide safer and more accurate screening for detecting pregnancies at increased risk for Down syndrome, trisomy 18 and open neural tube defects. Dr. Sylvie Langlois, Acting Medical Director of the BC Genetic Screening Program, stresses that it is important that each woman makes an informed decision about whether or not to have screening.

Older mothers have a higher chance of conceiving a baby with either Down syndrome or trisomy 18. The chance of having a Down syndrome baby for a woman of 25 is 1 in 1,250 while for a woman of 45 this chance rises to 1 in 24. People with Down syndrome have mild to moderate intellectual delays and have a higher possibility of having other health problems. There is no way to know prenatally how serious these health problems will be.

Trisomy 18 is less frequent and occurs one tenth as often as Down syndrome for women of all ages. Trisomy 18 pregnancies often miscarry. If a baby with trisomy 18 is born, the life expectancy is very short due to serious heart and brain defects.

Open neural tube defects occur in about 1 in 1,000 births regardless of the age of the mother. These defects occur when the brain or spinal cord does not form properly. When this defect involves the spinal cord, it is called spina bifida which can result in both physical and mental disabilities. When the open neural tube defect involves the brain it is called anencephaly. Babies born with anencephaly will be stillborn or die shortly after birth.

Genetic screening is done by testing the mother’s blood at two specific points in the first half of pregnancy. An accurate gestational age is required to ensure the most accurate screening results. If the mother is not sure of her last menstrual period because she has short/long or irregular cycles, or conceived soon after stopping oral contraceptive pills or while breastfeeding, a “dating” ultrasound will provide the best estimate of the gestational age of her pregnancy.

The first genetic screening blood test is done between 10 and just under 14 weeks of pregnancy, and the second blood test is done between 15 and just under 21 weeks of pregnancy. The woman’s physician or midwife will receive the screening results within 10 days of the second blood test. The testing of blood samples is done at the BC Prenatal Biochemistry Laboratory at Children’s and Women’s Health Centre in Vancouver. If a mother misses her first blood test, the second test can still be done. It is best however to have both blood tests because this improves the accuracy of the screen result.

When a mother receives a “screen positive” result, her physician or midwife will explain what this means and what further testing is available. A “screen positive” result does NOT mean that the baby has Down syndrome, trisomy 18 or a neural tube defect. It indicates that there is a slightly higher chance that the baby may have one of these conditions. In fact, most women who have a “screen positive” result do not have a baby with one of these conditions. After discussing the test results with her doctor or midwife a mother can then decide if she wants to have further testing.

If the screen positive result indicates an open neural tube defect, the mother will be offered a detailed ultrasound and an appointment with a maternal fetal medicine doctor or a genetic counselor at a genetic clinic in Vancouver or Victoria. This ultrasound can usually detect if the baby has an open neural tube defect.

If the screen positive result indicates Down syndrome or trisomy 18, a mother can have a diagnostic test that will tell for sure if the baby has one of these conditions. This test is called an amniocentesis and is done by putting a very fine needle into the mother’s abdomen to remove a few teaspoons of amniotic fluid from around the baby. The needle is guided by ultrasound and does not touch the baby. The baby’s cells that are contained within this fluid are examined to determine if the baby has Down syndrome, trisomy 18 or another genetic disorder. There is a 1 in 200 risk of pregnancy loss when an amniocentesis is performed. For this reason some mothers decline this test. Results from the amniocentesis are available within two to three weeks.

Mothers expecting twins may be offered a special ultrasound test called a nuchal translucency or NT test before 14 weeks. This test measures the fluid space at the back of the baby’s neck and will indicate whether there is a greater chance that the baby has Down syndrome or trisomy 18. Alternatively, the routine genetic screening blood tests can be done. Mothers of twins, who will be over the age of 35 at the time of delivery, will also be offered the option of amniocentesis without prior screening.

Any mother who will be over 40 when her baby is born will automatically be offered either chorionic villi sampling or amniocentesis. She may choose to wait for the results of the two genetic screening blood tests before making a decision about having this diagnostic testing.

Mothers who have an increased chance of having a baby with Down syndrome or trisomy 18 due to their age or previous pregnancy history will be offered a nuchal translucency (NT) ultrasound test between 11 and 14 weeks gestation. This is done in addition to the two screening blood tests.

Mothers with a personal or family history that increases the risk of them carrying a fetus with chromosomal abnormalities are offered specific diagnostic tests at an earlier point in pregnancy.

When prenatal genetic testing confirms that an unborn baby has Down syndrome, trisomy 18 or an open neural tube defect, the family can receive free genetic counseling services through medical genetics clinics in Vancouver or Victoria. A medical geneticist and genetic counselor will discuss the issues and choices with the family.

For more information about genetic screening and testing in B.C. visit www.bcprenatalscreening.ca.

Susan Miller R.N. BScN is a Perinatal Educator and Certified Breastfeeding Counsellor. She works with prenatal and post-natal families in the Greater Victoria area.