The Question: I’m early in my pregnancy and wonder what screening and diagnostic tests I should undergo. I’m worried about taking a test that leads to miscarriage. What is the best way to have a healthy baby?
The Answer: This is the type of question that Jon Barrett, head of maternal-fetal medicine at Sunnybrook, encounters frequently in his fourth floor office in the Women and Babies program.
“We generally recommend a screening test,” said Dr. Barrett. “They can be very confusing which one to choose. With genetic screening, people have to realize there is no way we can screen for all abnormalities.”
What parents worry about most – a baby being born with cerebral palsy – is not detectable through any test in the womb, according to Dr. Barrett.
Screening only helps detect the risk of having a baby with Down syndrome, a condition where a baby is born with an extra chromosome – usually an extra copy of chromosome 21 – making for 47 chromosomes, instead of 46.
The condition ranges from mild to severe. The heads of children with Down syndrome are usually smaller with a flat area on the back. Their eyes appear more rounded, instead of pointed. And the physical and cognitive development delays they experience growing up can be a source of frustration for them.
The test is not done because Down syndrome is the worst birth defect but because it is the most single common cause of detectable birth defects. There are others more severe – even causing death – but there are no screening tests currently to find them.
In Dr. Barrett’s experience, “most people do the first trimester screen, then if they are positive, go on to do amniocentesis.”
However, there are women who do not need to be screened: those who would not terminate a pregnancy under any circumstance.
For the remaining, here are the options:
• The integrated screening test is the nuchal measurement plus a blood test done once at 12 and again at 16 weeks that measures the three placental hormones. When the mother’s age is also rolled into the equation, the clinical team is able to assess the risk for having a child with Down syndrome. While the test is considered more accurate, the results are available at 17 or 18 weeks.
• Amniocentesis. This amniotic fluid test is not a screen but a diagnostic test done at 16 weeks gestation. Since amniotic fluid contains fetal issues, the fetal DNA can be examined for genetic abnormalities, and is 99.9 per cent accurate. Since it is an invasive test, there is a risk of miscarriage –ranging from 1 in 400 to 1 in 1,000, which in some cases is largely dependent on the pair of hands doing the test.
• Chorionic villus sampling involves removing a tiny amount of placenta tissue from the womb at 12 weeks gestation. It can be done through the cervix with a plastic tube or through the belly with a needle but either way, the loss rate is even higher, roughly double that of amniocentesis – for a 1 out of 200 risk of miscarriage.
Sunnybrook also has a special clinic that can do an anatomy scan at 12 and 14 weeks that is comparable to the one done at 18 weeks gestation.
“You need the right machine and the right probe,” notes Dr. Barrett.
In the future, there will be a blood test that takes fetal DNA in the mother’s blood, boasting 99 per cent accuracy.
That, said Dr. Barrett, is expected to replace all these other tools – though it will likely initially come at an out of pocket cost to patients to pay for the test, likely some time within the next year.
“In the future we will hardly have to do any of this to detect Down syndrome,” said Dr. Barrett. “Thus, while it may not detect more babies with Down syndrome, it probably has a lower false positive rate.”