Emily Oster has a book coming out entitled Take Back Your Pregnancy. In preparation for the book


launch, as most authors do, she wrote a nice piece in the Wall Street Journal about decision-making, risks, and how some of what OB/GYNs recommend during pregnancy isn’t always evidence based. In particular, she mentions listeria, a bacteria that is a food borne illness with potentially devastating outcomes during pregnancy. Osher quotes the risk of about 1/8,000 and felt that she should cut out queso fresco, but was still ok with eating deli meats.

In response, the NY Times Motherlode blog featured a beautifully written post by KJ Dell’Anotonia that picked up on the listeria risk that Osher quoted at about 1 in 8,000. Ms. Dell’Antonia was the one.

This led me to give thought to what is a valid discussion of information and risk and what is fear mongering? Listeria is actually a great subject matter for the discussion.

Listeria monocytogenes is a bacteria that is food borne. It is killed by pasteurization. In the United States listeria is typically acquired from eating unpasteurized foods or deli meat (studies of what grows in the deli counter are pretty gross, and as an aside I never buy deli meats, but that’s me).

Listeria is not a benign infection. The case-fatality ratio is 21%, although most people who get very sick or die from listeria are over 65 or have a weakened immune system.

Listeriosis (infection with Listeria) is 18 times more common during pregnancy. Since listeria can cause miscarriage, stillbirth, and serious illness for the mother and newborn and since it is significantly more common in pregnancy it is a valid topic for an OB/GYN to discuss during pregnancy. In fact, if you developed listeriosis in pregnancy and had a bad outcome and your doctor never mentioned listeria you could probably sue your doctor. In medicine an 18-fold increased risk is a lot.

But what does an 18-fold increased risk really mean? Can we translate that into more useful numbers? There are about 4 million births a year and the CDC reports about 114 cases of listeria a year during pregnancy. By my incredibly non scientific method the risk of having listeria that will be identified during pregnancy is about 1 in every 40,000 births. Turns out I’m not too far off as a recent review article quotes 1.2/10,000 during pregnancy, basically the 1/8,000 quoted by Osher.

Risk means different things to different people. When I was pregnant with triplets I knew based on statistics that the average gestational age is 32 weeks and the odds that I would take 3 babies home from the hospital was 87%. If I had a selective reduction the average gestational age for my now twins would have been 36 weeks and the odds that I would take both home from the hospital was 88%. Difficult stats. High stakes situation. I chose not to reduce and of course I was in the 13% who loses a baby and was on the wrong end of the gestational bell curve delivering my surviving sons at 26 weeks. However, I don’t regret my decision one bit because I made an informed decision.

When I discuss risk I focus on major complications and I typically use an air travel analogy. If the risk of a major complication with a procedure is 5%, I tell people to think about it like air travel. Would you get on an airplane if the risk of crashing were 1 in 20? It might depend on the reason for traveling and other factors. If you have cancer and the odds the surgery is going to save your life are 40% and your risk of dying without the surgery is 100%, then a 5% risk doesn’t look bad at all. However, if you have chronic pain and the surgery is to remove adhesions which we know will not help your chronic pain and the surgery has a 5% risk of a major complication then that 5% means something else. That 5% may also mean something more to you if you have a family member who died from a surgical complication or if you yourself had a complication from a previous surgery. Life experiences matter when deciding what risk you can tolerate.

So what about listeria? The facts are:

  • The risk of getting listeria is 18 times higher in pregnancy
  • If you get that infection during pregnancy there is a significantly increased risk of a bad outcome
  • Most pregnant women in America will not get listeria, but every year 120 or so will
  • There are some foods that increase the risk of listeria, unpasteurized cheese being the most common source in the United States but deli meats and other foods can be sources as well.
  • Nothing bad will happen if you avoid the most common sources of listeria for 9 months

How you assemble those facts will be different for different people. And an OB/GYN who has seen a case or two of listeria will probably present those facts differently than an OB/GYN who has never seen the infection.

Let’s put it another way. Knowing the risk of listeria is 18 times higher during pregnancy, albeit the absolute risk is low, do you think it would be appropriate for your doctor not to mention the increased risk?

Putting the fear of God into people is bad medicine and if your doctor makes you feel like you are a bad person for eating soft cheese during pregnancy, then you should probably get another doctor. However, it is good medicine to explain the risks of listeria and what can be done to avoid it. An informed patient listens to that information and then makes a decision.

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  1. p.s. What I tell my students when we discuss risk in class (I’ve taught stats twice while pregnant)– something that’s important is how difficult it is to avoid the risk. I tell them it’s easy for me to stop eating cold cuts and unpasteurized cheese. The chance of listeria is small, but getting it while pregnant is horrific, and it’s something that’s easy to avoid. So I don’t beat myself up if I accidentally eat something, but I avoid it while pregnant.

  2. Thanks for this. I’m pregnant now, and while I don’t mind the listeria precautions and find them a reasonable trade-off, I have decided this time that I will eat sushi from the (very trustworthy) place I like. That risk is different for me for several reasons (mostly that I think the risk of bad outcomes is exceedingly small). I’m currently marshaling my arguments for avoiding the glucose screen this time, as it made me badly sick for several days last time, and I can’t do that with a toddler. (I have hypoglycemia, which in my case is much worse in pregnancy.) I’m shocked when I look at the actual data that has gotten translated into risk groups for gestational diabetes, both in terms of risk of having the disease (where at least one paper I read has data that says one thing and text that says another) and in terms of bad outcomes. (Lest anyone jump down my throat, I am perfectly willing to monitor glucose, just not to deliberately make myself sick to an unsafe degree again.) I am a little worn out by how many people treat any hint of risk in pregnancy as if it absolutely requires a given action, without regard for patient autonomy.

  3. Thanks for your info Dr. Gunter. Listeria is a group of bacteria. It is fatal infection in the pregnant woman. It infection spreads to the nervous system, the nervous system become loss of balance. This Infection during pregnancy may appear mild but can lead to stillbirth, premature delivery and infection of the newborn. So Listeria infection is very harmful for a pregnant woman.

  4. Thank you, as a pregnant woman who is highly skeptical of American fear mongering especially surrounding food , this has been a tough one to make heads or tails over. I found this very helpful, thanks.

  5. And how do I explain to my daughter-in-law that I think travelling to Bali for a 4 day holiday, at 34 weeks, is an unnecessary “risk”?

  6. Isn’t it likely that the number of cases would be much higher if doctors weren’t telling women about it and the women were eating more deli meat and unwashed vegetables?

  7. I am in Germany and ate Camembert yesterday either breakfast, it was a small amount and I simply didn’t think about it. It was a branded (Le Rustique) from France made with pasteurised milk. Do you have any idea how high the risk to my baby is as I am becoming evermore nervous.

  8. What are the risks of getting listeria from *pasteurised* soft cheese? The recommendations are contradictory – foodsafety.gov says pasteurised soft cheese are fine, the NHS says they’re not, and I can’t seem to find reliable information to make a decisions.

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