You may have seen the startling headline that New Warning: Morning-After Pill Doesn’t Work for Women Over 176 Pounds.

Some people have asked if that’s right. Well. According to the actual research (PDF):

  • Among women with a BMI under 25, 1.3% got pregnant after taking Plan B (its equivalent, anyway)
  • Among women with a BMI 25-29, 2.5% got pregnant after taking it
  • A BMI of 30 is where it crosses into “not statistically different from women not using EC,” at 5.8%

You’ll notice the research is reporting BMI (which is a height-to-weight ratio), while the headline declares a specific weight. I’ve been trying to figure out how they came up with 176 as the magical weight at which EC suddenly stops working, and I think they just used the “average” American woman’s height, which is 5’4″.

My mom is 5’4, so 176 would put her in the “not effective – use a different method” group. My sister is 5’7″, so at 176 her BMI would be 27, which puts her in the “higher risk but still significantly better than nothing” group. My boss is 5’10, so at 176 her BMI would be 25, which puts her right at the edge of the “very effective” group.

So there’s that.

Also but bear in mind that the researchers aren’t even trying to say anything about cause, and BMI doesn’t really help us theorize. Two women with the same BMI may be very different in their health status, body composition, fertility, hormones, etc. At 176, my body fat percentage would be around 26% because I have the lean mass of a male my height, while a woman with “average” lean mass would have a body fat percentage of more like 33%. How lean mass interacts with emergency contraception, we have no idea at all.

So there’s that, too.

We don’t know what it is about BMI that’s causing the drop in efficacy. We can make up stories until the cows come home – like, it’s a simple mass thing: greater size = more dilute dose; it’s a hormone thing to do with the hormone production of fat we assume people with a high BMI have; it’s a behavioral thing – um, higher BMI women are more likely to have food in their stomachs, which slows down absorption of the medication; maybe women with higher BMIs are more reluctant to obtain EC because they face more overall stigma and so wait longer to take it. We just don’t know.

What to do about it? If your BMI is under 30, don’t worry about it. If your BMI is over 30, use a different EC method like the copper IUD. And of course everyone can do a lot by using effective contraception correctly and consistently whenever possible. Because there’s no prevention like primary prevention.

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