As you might imagine, I love talking to women about their experiences with hormonal contraceptives. And given my penchant for nerding out on details that others might find TMI (“…did your orgasms return to normal once you switched the brand you were on??”), I find myself talking to women about these experiences a lot. Like, A LOT, a lot. And one of the things that women most-often ask me about in these discussions is whether their hormonal birth control might be causing them to gain weight. Because many women swear that they gain weight on the pill, even when their doctors tell them otherwise.
But that conversation may be changing soon.
To start with, women’s doctors haven’t been lying to them. For a really long time now, the majority of studies that have set out to look at the relationship between hormonal contraceptive use and weight gain have found no relationship between the two. There have been some exceptions to this (I’m looking at you, hormonal contraceptive shot), but most studies fail to find a relationship between being on hormonal contraceptives and weight gain. This, of course, begs the question of why science hasn’t been able to document pill-induced weight gain when so many women report that this is what happened to them.
One possibility is that women are simply mistaking weight gained for others reasons on their birth control. It’s easy and convenient to blame our birth control for any problems we might have (although, please note that I have learned the hard way that the IRS will not accept “unusual birth control side effect” as an acceptable explanation for late taxes). So, it’s possible that women are misattributing regular, run of the mill weight gain to a birth control issue.
Another possibility is that some women *are* gaining weight on their hormonal birth control, but that science just hasn’t been able to capture it yet. This is the sort of thing that can happen when a medication impacts different people in different ways. Which of course it will. I talk about this issue a lot in my book. No two of us are built the same way, which means that each of us might respond to the exact same medication in very different ways. Especially something that influences sex hormones. This is why you should always listen to what your body is telling you. If you are experiencing a side effect that isn’t described in the package insert of your birth control, there’s a pretty good chance that you aren’t imagining things. Science probably just hasn’t been able to capture your weird side-effect yet because not everyone has it. Thankfully some new research is finally beginning to underscore this super-important point by showing gene-based differences in the risk of weight gain from the birth control implant.
In a new study that is in press in the research journal Contraception, researchers found that women who went on the birth control implant (which uses the third-generation progestin etonogestrel), at the time of the study, had gained an average of seven pounds since the time it was inserted (roughly two years prior). This is significantly greater than the average two-year weight gain found among women in the absence of implant insertion. More interestingly than that, though, was what they found next. Women who had two copies of the ESR1 rs9340799 variant of the estrogen receptor gene gained, on average, gained 30.8 pounds more than all other participants after going on the birth control implant.
Taken from Lazorwitz et al., in press. Note that the Y-axis shows weight as kg (instead of pounds).
This is huge. And it’s huge for a couple of reasons that may be meaningful to you even if you have no intention of ever going on the birth control implant.
The first reason that these results are pretty huge is because this study found that being on the birth control implant, for all of the women in their study, was associated with an increase in weight over time. Although most women gain a little bit of weight over time, regardless of how they are preventing pregnancy, seven pounds over two years is greater than what would be expected in the absence of a manipulation (here: the birth control implant). So, the implant appears to be associated with an increased risk of weight gain for all women.
The second thing that is a big deal is that how women responded to the implant depended on their genes. Women with the ESR1 rs9340799 variant of the estrogen receptor gene were likely to gain a whole lot more weight after going on the implant that were women with other genetic variants. Like, THREE TIMES more. This is important for a lot of reasons. First, it provides evidence that women can respond very differently to the exact same type of hormonal birth control. And this includes differences in the likelihood of gaining weight from being on it. Science has finally heard you. Some women will gain weight on the pill. Some women will lose weight on the pill. And some women will experience no changes whatsoever. And one of the factors that influences which camp you will fall into is your genes.
However, the biggest deal with this is research is that it has begun to expose the tip of the iceberg. This is one genetic SNP and one type of birth control. Imagine how many unidentified risk factors for unpleasant side effects (including weight gain) each one of us has latent in our genomes. There are probably side effects that you are vastly more likely to get than others because of your genes. And there is a chance that the existing research literature – most of which doesn’t look at gene-based differences in responses to birth control at all – doesn’t yet know about it. You aren’t crazy. You aren’t imagining things. It could be your birth control.
I am so excited about this next phase of research on the birth control pill. And I am excited that my lab and the research we are doing gets to be part of it. I look forward to seeing you all, your daughters, and my daughter being given access to better, more targeted means of fertility regulation.
Take care and be well.
PS: If any of you are interested in learning more about your genes and which ESR1 gene you have, you can actually download the raw data from your entire genome from 23 and Me. From there, you can dig around in your own genome to find our what you are made of.
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