One virus causes more birth defects in American babies than any other nongenetic disease. But most don’t know it exists.

Megan Nix first learned about congenital CMV when her daughter was diagnosed in 2015.

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One in every thousand babies born in the United States will have some symptom or long-term health problem due to congenital CMV.

But most doctors don’t discuss it with expecting mothers during routine prenatal care. Why?

Today, On Point: A mother’s journey to understand her daughter’s diagnosis. Stay with us.

CHAKRABARTI: So give us a little bit of background on CMV or cytomegalovirus. What exactly is it? I mean, it’s a viral infection, but how would we describe what it is?

PERMAR: Yeah. So I describe it as the most common infectious cause of birth defects and brain damage throughout the entire globe. It is a very common virus that most of the world, as adults and even in childhood, acquire. And in underdeveloped regions, most women have the infection before pregnancy. But in more developed regions, about half of women will have the infection prior to pregnancy.

CHAKRABARTI: And in children, not newborns, but in children and adults, how does a CMV infection present?

PERMAR: It’s often asymptomatic. So it’s a virus that many of us carry but don’t ever know when we were infected. It’s one that’s passed very easily with contact with body fluids. So close contact is sort of how it’s best transmitted. It’s also transmitted through breastfeeding, transmitted often by toddlers with how they like to share their fluids and also through sexual activity.

CHAKRABARTI: And so I’m seeing here, this is from the 2015 ACOG guidance that was put out, ACOG again being the American Congress of Obstetrics and Gynecology. And it says that the prevalence of CMV, whether it’s primary or secondary infection, really varies geographically, and actually by socioeconomic status. Like you were talking about a little earlier … it could range from .7% to 4% of pregnant women. Does that make sense to you, that big broad variance?

PERMAR: Yes, it’s very dependent on what one’s exposure risk is during pregnancy. So if you’re someone who lives with many other CMV infected individuals, which is more common in certain geographic areas, it’s more common. In lower socioeconomic status individuals and minorities and in immigrant populations. So those groups are often more exposed to CMV during pregnancy.

However, they may have been more likely to have the infection already before pregnancy and therefore have some immunity. But a really risky time is when a woman has no immunity to CMV before pregnancy and then is exposed for the first time during pregnancy. But often second or third time moms can be in that category because they have toddlers who may have picked it up from other toddlers. And then also people that work with young children like teachers or daycare workers are also highly at risk.

CHAKRABARTI: Yeah. Okay. Now, Megan, I’m going to presume that when you were having your prenatal visits while pregnant with Anna, I mean, you kind of said it, right, that this potential infection, CMV, never came up in conversation. Right?

NIX: Right. Never, never uttered by a doctor. I was never tested for CMV before or during pregnancy, so I don’t know that I had ever contracted it before having Anna. I assume that I contracted a primary case of it because she was born symptomatic, and a primary case will cross the placenta about 3%0 to 40% of the time. Whereas in these other populations that Dr. Permar is talking about, where women generally have already had a CMV infection prior to pregnancy, a reinfection with a new strain will only cross the placenta about 1% of the time.