A few years ago, I got myself the Implanon contraceptive implant. I was extremely happy with it, and after its three-year lifetime expired, I got myself a replacement.
I got it about 12 months after it was released in Australia, and only a few years after the clinical trials had been carried out. I remembered reading the information sheets and talking to my doctor about failure rates, and at the time, the only information was from clinical trials. In those, the only prenancies had been where the implant wasn't properly inserted (read: not at all) or where the woman had already been pregnant.
I thought I'd take a look and see if there was more data now, and I found this article through PubMed.
It gave me the information I was looking for - the rate of failure of the implant itself is extremely low compared to pretty much any other form of contraception (13 women in the study, which means around 1 in 1000 insertions - only NuvaRing seems to have a rate this low). But it also contains the following comment, which was a throwaway line - it wasn't followed up in any way in the rest of the article:
Interestingly, one woman in this group was reported to have gained 10 kg in weight since insertion of the device.
Sigh. It seems to me that that little fact is only interesting if you can compare it to how many women are likely to put on 10 kg in weight in a three year period generally. My guess is that if you take any truly random group of 13 women, it wouldn't be particularly suprising to find that one of them has put on 10 kg in the last three years!
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