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AN ARTICLE HOSTED BY IMPREGNORIUM.NET PREGNANCY TIMING Let’s stay with women and fertility and something rather fundamental to the process of falling pregnant: having sex. Studies from a leading research centre in the United States have found that women’s most fertile period isn’t what the experts used to think, and, what many women know already, their sexual drive is significantly biological and geared to their chances of conceiving. Dr Alan Wilcox is an epidemiologist at the National Institute of Environment Health Sciences in Durham, North Carolina. Alan Wilcox: It’s actually an accidental spin-off of a study we did for a very different purpose. The study originally was intended to explore the factors that influenced early pregnancy. We enrolled women in a prospective study before they got pregnant, and asked them to collect daily urine specimens so that we could study their hormone activity in early pregnancy, and also identify very early pregnancy loss. Norman Swan: In other words, this is the whole problem of really identifying why some women become pregnant in the first place, why not, what are the factors, and are more women losing babies than they might imagine through what they thought were late periods, for example? Alan Wilcox: That’s right. And so as part of that study, we had to collect pretty careful information about when women had intercourse, and when their menstrual cycles were, and then from the hormones that we looked at in the urine, we were able to identify when their day of ovulation was, and from that we were able to estimate the fertile days of the menstrual cycle, and in fact another spin-off of this study was the identification of six fertile days in every menstrual cycle, that women could actually conceive during, if they had intercourse on those six days. And those are the five days before ovulation and the day of ovulation itself. And so that was something we kind of found on the way to this more detailed exploration of fertility in early pregnancy; that six-day window has since been confirmed in other studies, so we felt like that was on pretty good grounds, and so that took us to this next little side issue of the patterns of intercourse. Norman Swan: So tell us about that. Alan Wilcox: There’s actually a story behind the story here, because what we noticed, looking at our women who were trying to conceive, is that there was this sharp increase in the frequency of intercourse right in this window that we identified as their fertile days. And on the one hand we were pretty sure that women weren’t doing this deliberately, because for one thing, before we did this study, the understanding of when the fertile days was, was a little different. Most of the textbooks said that women were still fertile a couple of days after ovulation, and so we thought – Norman Swan: They’d have been timing it around ovulation or just afterwards, rather than the run-up. Alan Wilcox: Right. That we would have seen more intercourse after ovulation, and what we actually saw was intercourse dropped abruptly at the time of ovulation and became quite low on the day afterwards. So we thought we had something that was kind of interesting, but we couldn’t convince ourselves that it was not intentional on the women’s part. And so we had collected the comparison group for our pregnancy study of women who had been sterilised, women who could not be pregnant. Norman Swan: So although they were sterile, they still were menstruating and still had the ups and down of the regular hormonal cycle. Alan Wilcox: That’s exactly right. And presumably not paying any attention to their timing of their intercourse with regard to ovulation. Norman Swan: They’d kind of given up. What did you find? Alan Wilcox: So we went back to those women and looked at their patterns of intercourse and to our great satisfaction and I might say relief, since it was a whole lot of work to try and replicate this, we found quite the same pattern, that the highest frequency of intercourse was exactly during the six days that we were identifying as the fertile days, even in these women who were themselves were sterilised. Norman Swan: How much more frequent was it? Alan Wilcox: Well not a whole lot in one sense. The increase in frequency – Norman Swan: So people weren’t going at it like rabbits in the six days beforehand. Alan Wilcox: No, it’s not as though if was an all-or-none thing, the increase was about 25% over the average, over the cycle. But that was still enough to be not just statistically significant, but probably biologically significant. Norman Swan: It was enough to become pregnant. Alan Wilcox: Before we looked at these data, the general assumption among people who were interested in fertility is that intercourse was random, with regard to ovulation or fertile days, in the absence of any intentional trying on the couple’s part. And so you might expect that if a couple who was not meaning to get pregnant and just happened to have unprotected intercourse at some particular time because they got carried away on that particular occasion, you might expect – Norman Swan: So it was like hitting a moving target, in a sense. Alan Wilcox: Right. You might expect it was just sort of random, and they wouldn’t have any more chance of getting pregnant than you expected just by a random shot. But what our data suggest is probably on those occasions when couples get carried away and don’t use precautions when they should, it’s also very likely it’ll be just the time when they’re likely to be fertile. Norman Swan: And presumably you’re saying this is biological, there’s something in the hormonal state which increases the likelihood of intercourse taking place. Alan Wilcox: Yes, we’ve really scratched our heads to come up with any other explanation, and we don’t see one. Norman Swan: Did you take a behavioural diary during this time? In other words, measuring women’s libido, for example, during this period? Alan Wilcox: We did not. And that obviously is the next thing that would be a very interesting study to do. Norman Swan: Because women will tell you that their propensity to wanting sex does change during the menstrual cycle.
Alan
Wilcox: Yes, and there has been some research
on that, just talking
Norman
Swan: So they’re not necessarily matching
your six fertile days Alan Wilcox: Well we did find a couple of papers that suggested that women were more interested in sex around their ovulation, but then there were some other papers that suggested the opposite. And I think partly those papers, it’s not the women’s fault, I think that the studies themselves are sometimes based on very imperfect estimates of day of ovulation for example, or asking women to remember back over previous time, rather than keeping daily diaries, so there’s a lot of ways I think to go wrong in this kind of study, if you’re not very careful.
Norman
Swan: Now it takes two to tango, and there
are some fascinating Alan Wilcox: Yes, I think there’s some good evidence that that may be going on. Of course the study of pheromones is one that has attracted a lot of attention commercially, and if somebody could get that into a bottle it probably would have quite a success. And there is some attempt to do that, although I think it’s kind of in the early stages, but I think your general point is very well taken, that there could be things that affect the man’s behaviour as well as the woman’s. Norman Swan: Let’s move on to this, just finally, this six-day story, because as you say the received wisdom has been you’re not going to get pregnant until the egg’s released, and therefore ovulation and the day or two afterwards is where the money is, and what you’re saying is the money is really in the run-up to ovulation, where really it’s the outer limits of sperm survival in the female genital tract.
Alan
Wilcox: You’re right. What our data
would suggest is that the sperm can survive
for up to five days, and that the egg doesn’t
survive long at all if it’s not fertilised.
I think the most surprising thing in our
data that we Norman Swan: So that sperm have got to be there when the egg pops out.
Alan
Wilcox: That’s what it looks like,
and that was pretty clear what we were seeing
in our data, but one study isn’t enough
to prove this sort of thing. And there has
since been two other independent studies
that have Norman Swan: And this broader study of early pregnancy, what else have you found that increases or decreases the likelihood of successful conception around this time, rather than what you’ve called a side issue, but it doesn’ t sound to me like a side issue, the fact that the primary act which allows pregnancy to take place, happens at a critical time, and may be biologically driven. But then, that having happened, have you found other things related to the success or failure of early pregnancy?
Alan
Wilcox: That’s a really good question
and especially since I work at an institute
that is interested in environmental effects
on health. This is a Norman Swan: Dr Alan Wilcox is an epidemiologist at the National Institute of Environmental Health Sciences in Durham, North Carolina.
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