Ben Goldacre, The Guardian, Saturday 8th January 2011

“600 pregnancies despite contraceptive implant” said the BBC. “500 fall pregnant after having contraceptive implant” said the Express. “Contraceptive implant alert” said the Daily Mail: “Hundreds of women fall pregnant after birth control fails”.

The story first broke on Channel 4, and it’s still not entirely clear why it’s the biggest medical story so far this year. Some women have had some compensation: but a lot of people get a lot of payouts. There’s a law firm touting for more business, but that’s hardly news either, and the news story was: this contraceptive device has failed.

But is the failure rate exceptional? A figure means nothing if it has no context: 600 pregnancies sounds like a big number, but there is no way to know what it means unless we know how many women had Implanon, and for how long. The device was first launched in 1999, so that makes 60 pregnancies a year, which feels like a smaller number, but that’s still not enough information. The figure that epidemiologists use for context is “person-years-at-risk”.

I contacted the MHRA. They estimate that 1.355 million Implanon implants have been sold. If we assume that each implant lasted 3 years, which they do, this gives a total exposure time of 4.06 million women-years at risk. 584 unplanned pregnancies in this exposed population means there were 1.4 unwanted pregnancies reported for every 10,000 women with Implanon implants per year. If you prefer, you can say that the failure rate is 0.014% per year. This is rather good: in fact, implants are still the most reliable form of contraception.

Back with our 584 unwanted pregnancies, meanwhile, we see the difference between individuals and statistics. For some of the people who got pregnant, from their end of the telescope, this is a disaster. Some cases may well have been avoidable. Some will want financial compensation, and you’ll have your own views on the state’s role in this more broadly. But for you as a potential user of the implant, or a news editor, looking at the whole population, even though this particular variety of implant had a problem with insertion – which has already been improved on – at its worst, it still seems to be one of the most effective forms of contraception available.

Meanwhile there were 1,669 reports of suspected adverse events associated with the device, including arm scarring and other injuries. This is a rate of 4 adverse events for every 10,000 women per year, or if we assume that the implant lasts for 3 years, around 1 in every 1,000 insertions results in a suspected adverse event report.

Every medical intervention can have unintended side effects. It’s worth noting that for the past 5 years, patients have been able to report adverse events for any drug directly to the MHRA online, just like doctors have always done, and I encourage you to do so, as it provides an important resource for the fascinating problem of catching adverse reactions before they affect too many people.

Spotting a true signal of a problem in this database, incidentally, presents an awesome methodological challenge, because any one of thousands of drugs could cause any one of thousands of types of adverse reactions or symptoms. What you get, as a result, is a giant spreadsheet with a number in each cell, and inevitably, with such a vast number of cells, by the simple play of chance, there will also be a vast number of cells with an unusually high number of event reports in them, simply through random variation. Geniuses who worship the Reverend Bayes, with his statistical methods that exploit prior probability, have enjoyed scratching their heads over this resource for many decades.

And lastly, just like a number deserves its context, so too does a scare. In the 1990s, a temporary concern about a modestly increased risk of blood clot, particularly in one type of oral contraceptive pill, resulted in a mass abandonment of oral contraceptives generally, around the world, including among low risk women, and the following years saw an increase in both pregnancies and abortions, with all that this entails. Words can do harm, just as surely as hormones can.