Does Cervical Mucus Screen for Genetic Compatibility? (Episode 547)

Full Transcript: Episode 547

Lisa Hendrickson-Jack: Today I’m sharing a brand new episode in my FAMM Research Series, and the topic of today’s episode is genetic compatibility and the role that cervical mucus may play in this.

This is a topic that I’ve covered before in a different way. In The Fifth Vital Sign, in chapter seven — the pill chapter — I have a section called “Will the Pill Cause You to Choose the Wrong Partner?” When I wrote it, it was highly controversial, and I had people coming out of the woodwork from time to time suggesting it wasn’t a good idea to even talk about this. But the fact is that there is a whole area of science — evolutionary biology — where researchers are looking at some of these factors. The hypothesis is that we are designed in such a way that we select our mates based on genetic dissimilarity. If you look at several individuals, you are more likely to choose a partner under normal circumstances whose genetics are a lot more dissimilar to your own.

This is where the pill comes into the conversation. A lot of the research suggests that when you’re choosing a partner, your sense of smell — your olfactory senses — plays a significant role. The research suggests that when you are just living your best life, not on the pill, you are going to at least subconsciously make that decision based on the scent that you perceive from them. And what they found is that women on the pill — it changes how they perceive these scents. The research suggests that women who are on the pill are more likely to choose a partner whose genetics are more similar to theirs, which theoretically has implications that maybe they’re not choosing the same partner they would have had they not been on the pill.

There are anecdotal cases of women — some of whom I’ve spoken to, and I shared an example in the book — who met their partner on the pill, came off the pill at some point, noticed a scent they hadn’t noticed before, and in extreme cases their partner wasn’t as attractive to them as they had been before. This is scary information, and this is where the controversy comes in. You don’t want to boil down a person’s whole relationship to their scent. And of course, that’s not what we’re saying. But I think it is a useful exercise to look at what the research is saying, because if the pill is changing that natural inclination, we should know about it.

But in the study we are going to be talking about today, it’s addressing a different question. It’s looking at cervical mucus. Instead of looking at the olfactory senses and the concept of courting before the actual act of mating, they’re looking at potential genetic screening capability within the act of mating itself. The question they’re asking is: is the cervical fluid specifically screening out, or selectively screening, depending on the genetic similarity of your partner? This is a next level concept, and that’s what we’re jumping into today.

As we dive into the topic of cervical fluid and its potential capacity for genetic screening, I think it’s useful to talk about what we already knew about cervical fluid. In chapter three of The Fifth Vital Sign, I went into a lot of detail about cervical fluid and shared the basis for the fertility awareness method. When we look into the research on cervical fluid, we learn that it facilitates rapid sperm transport and selectively screens sperm of poor motility or morphology. It screens them out. Our cervical fluid has the capacity to screen the sperm coming in to ensure that only the best quality sperm actually make it through to our reproductive tract. That’s what we already knew.

But this paper takes it a little bit deeper and suggests that there’s potentially more to it than just motility and morphology. The mucus may have the capacity to screen for genetic similarity. The genes that the researchers are referring to are the MHC genes — the major histocompatibility complex immune genes.

Going back to the smell research — you might have heard of the T-shirt studies, where women smell T-shirts worn by sweaty men for a period of time and determine which they’d prefer based on scent alone. They find that women are more likely to select men whose genes are most different from theirs. The theory is that we are choosing partners who are more genetically different from us, which leads to more diversity in the gene pool and is associated with a stronger reproductive capacity.

So these researchers were looking at this question: is there an aspect of this taking place at the mating level? Is our cervical mucus actually playing a role in screening? Is it possible that if you have a couple who is very genetically similar, that could have a negative impact on the chances of conception? Could the cervical mucus actually be selectively screening out the sperm to a greater degree if you don’t have that optimal genetic match?

What they did in the study is they took cervical mucus from a variety of women, took sperm samples from a variety of men, and treated the sperm with the mucus of different women to see if the sperm would interact differently within the mucus of different women. They explain that cervical mucus contains a wide variety of immunological molecules, and that when cervical mucus is exposed to sperm, it triggers an inflammatory immune response that varies depending on the genetics of the male. That is what led them to question whether our cervical mucus is playing a role in selection based on the genetic similarity of male partners.

What they found was that when they exposed the cervical mucus of different women to different men, the genetic similarity or dissimilarity had an impact on what they called sperm performance. When they looked at how quickly sperm were swimming, their swimming velocity, their activation, their viability — just how the sperm were actually interacting within the mucus — they found that the performance of sperm in the cervical mucus was influenced by the genetic similarity or dissimilarity between the male and female.

This is really interesting research. It is a little scary, because it means there’s yet another factor that’s out of our control that could be having an impact on our fertility. But it does show this consistent idea that there is a genetic selection taking place — this concept of mate choice, going back to this idea that we are choosing partners at least somewhat related to pheromones. If we take it out of the weeds of the research and think about it logically: we’re all attracted to some people and not others, and we don’t always know why. Sometimes there’s this intangible thing. I think this type of research gives us a scientific basis for some of those things we can’t really put our fingers on.

It’s also highly controversial because it does suggest that there are couples out there who may not be the best genetic match for each other. And what’s difficult is there’s no way to measure this. There’s no test you can take to determine your genetic compatibility. My sense from the researchers is that by studying these types of interactions, it sheds light on the overall picture. The researchers went into a lot of detail about the role of cervical mucus more broadly — the same aspects I covered in The Fifth Vital Sign’s third chapter, just talking about how cervical mucus screens generally for sperm.

From the study itself: “Cervical mucus has been thought to play an important role both in sperm function regulation and in sperm transport through the cervix. The cervical mucus may simultaneously aid migration of functionally normal sperm and serve as a selective barrier for abnormal and poorly motile sperm cells.” The researchers also discuss how cervical mucus facilitates rapid sperm transport into the cervical crypts — the little folds inside your cervix where the mucus is made, but which also serve as temporary holding cells for sperm. I always use the example of a sperm hotel. These cervical crypts hold sperm cells for periods of time, and then when you’re nearing ovulation, the sperm are released into the reproductive tract in advance of ovulation. There are a lot of really cool things happening in your cervix — and of course it begs the question of why we were never taught this.

So what do we do with this information? One thing to think about — as controversial as it is — is that I’m certainly not reducing us to our genetics and saying that’s all that matters. But the research shows us that it is a factor. We all have a sense that there’s always a special something when choosing a partner that is outside of our logical mind. You can’t always provide a logical aspect to it. There are aspects of our genetics, our physiology, our senses, and how we interact with people that we can’t always explain. That’s one of the reasons I find this research fascinating — it gives us insight into some of those inexplicable things.

If I bring this back to the birth control pill for a moment — this study had nothing to do with the pill, because it’s just looking at the natural impact of genetic similarity or dissimilarity on the way sperm interacts with mucus. But if we take it to a broader level: when you look at the controversy around the pill and whether you could be choosing the wrong mate, essentially we have our natural way of being. When we’re naturally cycling, we have our hormone cycle and pheromones doing their thing the way we were designed. When we introduce the birth control pill, it changes that natural flow. It changes our natural reproductive hormone cycle. It seems to have some effect on our pheromones and the way we perceive our partners’ scent — which would indicate it could be throwing off our natural picker. Your picker to choose a partner may be affected by exposure to contraceptives.

This is where the controversy is. It doesn’t mean that if you were on the pill when you met your partner that you made the wrong decision. This is just a conversation about what the research is suggesting. That is only one of the factors that would come into play when choosing a partner. But it is food for thought: what if having 100 million women on contraceptives throughout their reproductive lives is playing a role in mate choice? Could it impact fertility?

The researchers are not able to make any conclusions about how this could impact future fertility. But what they’re suggesting is that in some cases there could be genetic incompatibility, and the cervical mucus could theoretically be having a negative impact. If they could make that determination for a couple who’s been trying to conceive and there may be genetic incompatibility, it could lead to increased success for those couples — maybe they would have other options, like bypassing the cervical environment, or some other approach. By having this information and knowing it could be a factor, the researchers are suggesting this could give us insight into why some couples are struggling with infertility. The more knowledge you have, the more power you have. This could potentially lead to solutions for couples diagnosed with unexplained infertility where this could be one of the factors.

The perspective I bring is that if our natural choosing capacity is not altered, we’re probably naturally selecting the best partners based on that natural inclination — because it’s not something we have conscious control over. Naturally, we’re set up to be more attracted to partners who would be more suitable. The approach I take is: we don’t want to mess with our natural propensity to choose a partner. We want to lean into that natural selection, whether it’s the scent aspect or other pheromones. When we are in the stage of dating and choosing partners, it should be something we’re able to do naturally. It would be useful to think about that word of caution: if we’re on hormonal contraceptives, there could be some impact on how we’re choosing partners.

I know it’s controversial. And whenever you talk about this idea of mate choice and how the pill could be having an effect — or going deeper to say that our mucus could have a different effect on sperm of men who are less genetically compatible — it’s a very scary concept because it is very much out of our control. But the good news to fall back on is that when we are cycling naturally, we would naturally be choosing partners based on that natural selection process. If we’re cycling normally and our hormones are doing what they’re supposed to do and our cycles are healthy and all of these natural things are in place, we would naturally gravitate to the most suitable partners.

I try to be neutral related to the pill because I took the pill, and 80% of women take the pill at one time or another. It’s not to say no one should be taking it. But I do think we should be cautious about how and when we’re using it, and we should be aware that it could be having these different effects. It is worthwhile to consider when you are choosing a partner — not to say it’s the only factor, but the research suggests it potentially has an impact, even beyond just the scent, but potentially on reproductive capability in the future.

With that said, this is one study, and the researchers acknowledge that this is an area that hasn’t been studied as much — particularly this idea of cervical mucus and its ability to screen for genetic differences or similarities. It is a burgeoning field of research and more research is needed. I wouldn’t draw any firm conclusions, but I do think it’s useful to start that conversation. It is really fascinating to learn that cervical mucus itself could have a greater capacity for screening than what we had initially learned about related to just the general quality of sperm. I hope you found this episode interesting and intriguing.