Contraceptive failure figures: meaningless without comparisons

is a journalist, author and creator of the BBC Radio 4 programme More or Less

What's likely to happen as a result of today's contraception scare?

"More than a thousand complaints about Implanon have been received by the medicines watchdog ..." Channel 4 News

"Nearly 600 women have become pregnant despite using a popular contraceptive implant ..." BBC News website

It should depend on how fallible the contraceptive is in comparison to the alternatives.

But that's not reported.

Here's some data. I looked up everything that follows on Google. It took ten minutes.

The National Institute for Clinical Excellence reported in 2008 on the take up of various contraceptives. The report says:

"Contraceptives prescribed and dispensed in primary care in England in the 12 months to March 2007:

Implants (Implanon): 42,000."

The total has been rising steeply. In 2006, it was about 27,000; in the early part of that decade, between 10,000 and 20,000 a year.

Eyeballing the data and the charts suggests something over (perhaps well over) 300,000 prescriptions in the last 11 years; each prescription lasts three years.

That's about 900,000 woman years of implant contraception.

(The BBC story reports 1.4 million users of Implanon: that figure looks too big to me - I suspect an error here. But if true it would make my point more strongly.) 

There have been 584 pregnancies.

Taking my 300,000 figure, that works out as a failure rate - i.e. unwanted pregnancies - of about 0.07% each year.

This is astonishingly, breathtakingly good. A 1% annual failure rate would have been equal to 9,000 unwanted pregnancies in 11 years.

For comparison, in 1999, the Guttmacher Institute, which studies sexual health issues, reported: 

"When contraceptive methods are ranked by effectiveness over the first 12 months of use (corrected for abortion underreporting), the implant and injectables have the lowest failure rates (2-4%), followed by the pill (9%), the diaphragm and the cervical cap (13%), the male condom (15%), periodic abstinence (22%), withdrawal (26%) and spermicides (28%)."

And the journal of the Association of Reproductive Health Professionals reported in 2008 a failure rate among those who achieve "perfect" use of implants of 0.05% - pretty much in line with my own crude estimate.

This is reportedly twice as reliable as male sterilization and ten times as reliable as female sterilization. Perfect use of the combined pill and progestin-only pill is reported to result in six times as many failures as the implant.

Now maybe my estimate is inaccurate. It's a back-of-an-envelope scratch around to try to get a sense of scale. And maybe something else is going on. If these numbers are right and relevant, it's interesting that the NHS pays out compensation at all. Does it also pay out for the pill? Makes me wonder if there's something we don't know. 

So report the failure by all means, and report the payouts. But is there any chance that any of those reporting the story will offer any context that is helpful to women (and men) who want to make a decision about contraception?

Or do they just want to scare them? Again, the question: what's likely to happen as a result of today's scare about the awful ordeal of the unwanted pregnancy?

I'll tell you my guess: more unwanted pregnancies. 


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