Congrats on all the new sonograms you got this week! Is there a whole string of them dangling down your fridge? If so, use this week as your first lesson in trimming your photo collections, lest you become one of those parents batch-uploading weekly photo shoots to social media. Pick the clearest image and add the rest to your fetus’ surprisingly large box of stuff. Also, if this is not the first fuzzy image you’ve displayed for loved ones to see, here’s a good book on how to take great photographs.
In the majority of pregnancies, the first ultrasound lets you know that the pregnancy seems to be progressing well. That first ultrasound also likely comes with your fetus' first report card, and with that report can come scary-sounding numbers.
One of those scary numbers is your chance of carrying a fetus with Down syndrome. Visit any online pregnancy forum and you’ll see women reporting some very specific numbers, along with their panic of a baby with “Down’s.”
Before we dive into the numbers, let’s get our terminology straight. The correct term for the genetic condition you're concerned about is Down syndrome. (Down is the last name of the doctor who identified the condition, but he doesn’t *own* the syndrome, hence the lack of apostrophe.) The National Down Syndrome Society’s preferred language guide can help all of us approach questions about Down syndrome more accurately and sensitively.
Risk of Down syndrome is first calculated by maternal age, because the risk increases dramatically with age. A woman aged 23 is presumed to have a 1-in-1429 risk of delivering a baby with Down syndrome, while a woman aged 43 is presumed to have a 1-in-49 risk. These risk calculations stem from a 1983 paper in the Journal of the American Medical Association. If you happen to have academic library access, by all means read the paper, but you can also find the numbers in popular obstetrics textbooks or in Oster’s Expecting Better.
Those above risks are for all women. A woman’s individual risk will change with more information, including the date of conception, the nuchal translucency (fluid behind fetus’ neck), and hormones in the mother’s blood. That’s why women who receive a first trimester screening exam often receive a specific number (1-in-91, for example). The risk hasn’t actually changed, but more information has allowed genetic counselors to provide a more accurate risk assessment.
The problem with these numbers is that their specificity seems to unnecessarily panic people. Let’s say you that, after the first trimester screening, you meet with a genetic counselor who tells you that you have a 1-in-91 risk of chromosomal abnormality. The report you receive describes your risk as “high."
After hearing that news, you could spend the next several days panic-googling and posting questions to pregnancy forums. Or you could recognize that when it comes to pregnancy, expectant parents are terrible risk assessors. In one study of pregnant mothers in Munich, researchers found that many women severely overestimated their risks of delivering a baby with Down syndrome. 67% of women in the study estimated their risk more than double their actual risk. Other studies have demonstrated that when people hear risks phrased as “1-in-x,” they tend to see themselves as the 1 and not the x.
My main advice to you this week is to turn “1-in-x” statements into percentages. If it has been a while since your last math class, just remember that “percent” means “per 100.” To find a percent, divide your numerator (the top number) by your denominator (the bottom number) and then multiply by 100.
So, in the case of that 1-in-91 chance of chromosomal abnormality, you would calculate your risk this way:
(1/91) * 100 = 1.1%
If you have a 1-in-91 chance of chromosomal abnormality, you have a 1.1% risk of a chromosomal abnormality. In other words, you have a 98.9% chance of no chromosomal abnormality.
Even when the "x" numbers get smaller, many of the "1-in-x" numbers you'll see reported throughout your pregnancy are needlessly terrifying:
A 1-in-75 chance of a problem = 98.7% chance that everything will be fine
A 1-in-25 chance of a problem = 96% chance that everything will be fine
A 1-in-10 chance of a problem = 90% chance that everything will be fine
A 1-in-2 chance of a problem = 50% chance that everything will be fine
Understanding percentages makes you better at understanding risk, which will help you make better decisions throughout pregnancy and beyond.
But let's say you’re still panicked about that 1-in-343 risk statement you received from your genetic counselor. You're restless with that 99.71% chance of everything being fine, and you certainly aren’t alone. A whole subset of researchers study medical decision making, and a smaller subset of those researchers specialize in how to communicate risk to patients. They’ve found that when it comes to communicating risk, it helps to draw a picture.
One study found that participants were more likely to correctly remember risk when it was displayed in pictographs resembling people. You aren’t likely to receive this kind of risk pictograph at a genetic counseling appointment because most people’s Down syndrome risk is so incredibly small that such pictographs are impractical. If your genetic counselor presents you with a 1:500 chance of Down Syndrome, to make an accurate pictogram you’d need 499 gray baby icons and one blue one to depict the risk.
You’re unlikely to get such a pictogram from your genetic counselor, but you can draw yourself some quick reassurance by making your own. Grab a pencil and draw yourself 499 babies. Then grab a blue pen and draw one more. That blue baby represents your overall risk. The fact that your hand hurts from drawing all those other babies can help show you how low your risk is.
Still panicked about tiny risk probabilities? It may help to read a few non-mathematical perspectives on chromosomal abnormalities, including this slate piece about prenatal testing and personhood as well as this welcome to Holland.