The maker of the European version of the morning-after pill announced in late November that women who weigh more than 165 pounds and are in need of emergency contraception shouldn’t count on their pill to work. This revelation has big implications for the US since the US manufacturer of Plan B One-Step acknowledges that both types of emergency contraception rely on the same active ingredient, levonorgestrel, to prevent pregnancy; so do the generics. Heightening concern is the fact that, unlike in Europe there’s been no change to the warning label in the U.S., leaving many unaware they may be in for a very big surprise.
At Book of Odds, we maintain a database of the odds which affect people in their everyday lives. This includes extensive information on risks but also the odds of everyday life, familiar likelihoods which can be used for comparison and to aid in understanding probability. We decided to look at published data and our own to determine how many women might be affected. The result was so impressive, we found ourselves surprised this story hasn’t gotten far more attention from news organizations.
One legitimate reason for the scant media coverage is that key numbers are missing from the story, making it hard to give it accurate context and appropriate importance. How many women and their partners are at risk? To know that we need to know how many women are of childbearing age; of those, how many are over the weight threshold—and how is that threshold most accurately defined? Finally, are some groups of women more at risk than others?
Let’s start with some background. While legal wrestling regarding the over-the-counter status of the drug and the legal age to purchase made the news, research was being done at the University of Edinburgh by Anna Glasier and her colleagues on its effectiveness. This work, published in Contraception in 2011, showed a risk of pregnancy three times higher in women who were obese. Since obesity is determined by body mass index (BMI), the first findings were framed in these terms. HRA Pharma, the French pharmaceutical company which sells the pill in Europe, was prompted to take another look and uncovered something curious. The correlation of ineffectiveness was stronger to absolute weight, no matter what the height of the woman, than BMI, a relative measure. When the HRA Pharma put warnings on the packaging of Norvelo, its emergency contraceptive, it used the absolute measure: It warned that the drug is completely ineffective for women who weigh more than 176 pounds and begins to lose effectiveness in women who weigh more than 165 pounds. It is not recommended for anyone over 165 pounds.
Mother Jones jumped on this story in the U.S. and followed it conscientiously to get more details. As a way of expressing the magnitude of risk, the original piece referenced a CDC study on weight distributions and the Edinburgh study using BMI/obesity. Molly Redden, who covered the study for Mother Jones, found a clever way of conveying the enormity of the issue. She pointed out that the average weight of US women 20 and over was 166.2 pounds, above the warning weight of 165. She also highlighted the issue of race in this data, pointing out that ‘the average weight of non-Hispanic black women aged 20-39 is 186.2 pounds.” This approach gives the right sense of scale but no estimate of the number who might be at risk.
As anyone knows who dreads stepping on the doctor’s scale, how much of a lecture you are about to receive depends on both your height and your weight—a weight that’s borderline for someone who is 5’10” is entirely too much for someone who is 5’0”. That’s because what the doctor’s most concerned with is your relative, not your absolute, weight. When interviewed by Mother Jones, the HRA Pharma CEO was firm that the risk women face in taking the morning-after pill has to do with the number on the scale, not BMI. "We were surprised,” she said, “but the findings were really quite striking from a statistical point of view."
HRA Pharma warns that a higher dose of the active ingredient doesn’t increase the efficacy. A woman of any height or BMI who weighs 176 pounds needs to know Plan B or levonorgestrel will not prevent an unwanted pregnancy. Popping an extra pill, as one might be tempted (though ill-advised) to do will not change this fact. And if a woman weighs between 165 and 176, she shouldn’t be confident the morning-after pill will be effective. So how many women need to get this warning?
There are 70.6 million women of childbearing age in the U.S., which the CDC defines as between 16 and 49. Using the CDC percentiles, Book of Odds estimates that 1 in 2.6 women of childbearing age or 7.9 million women, weigh 165 pounds or more. Those are about the odds that an adult skips breakfast. One in 3.4 women, or 6.1 million, weigh 176 pounds or more. Those are about the odds that an adult brushes his or her teeth daily. (These estimates are based on 2011 population and National Health Examination Survey weight distribution data from years 2007 to 2010.)
The odds that the morning-after pill will not work are higher for black women than for white women and Hispanic women. As indicated in the table below, 66 percent of black women between 20 and 39 weigh 165 pounds or more, and 71 percent of black women between 40 and 49 weigh 176 pounds or more. The morning-after pill will have a higher rate of failure among black women.
But how many women actually use the morning-after pill? It is hard to get the exact number from public records since there are variations in age ranges and years studied, but it is possible to get a rough estimate. There are about 70.6 million women of childbearing age, and roughly 75 percent of these are sexually experienced, or about 52 million. The odds a woman in the sexually experienced group will use emergency contraception are 1 in 8.2 or about 5.8 million.
What we don’t have is the data to know if the weight distribution for women who are sexually active is the same as that in the general female population, or if it varies for women who chose to use emergency contraception compared to those who don’t. If, however, we assume those weight distributions are the same, approximately 2.2 million women a year may be relying on a morning-after pill that won’t work. Sounds like it’s time for a new warning label—and for the media to spread the word.
Amram Shapiro is the founder and Louise Firth Campbell the COO of Book of Odds. With Rosalind Wright, they are the authors of Book of Odds, From Lightning Strikes to Love at First Sight, which HarperCollins is publishing on January 7, 2014. It is “a numerical snapshot of the U.S.”