Wanting to start a family the natural way? For a healthy 45-year-old woman, you may be in for a five-year wait.

That's the conclusion of Richie Cotton, a UK-based data scientist, who discovered when he and his girlfriend wanted to start a family that statistics on how long it takes to get pregnant are hard to come by. The National Health Service in the UK does provide statistics on the "monthly fecundity rate" (MFR) — the chance of becoming pregnant in a month. This varies by age: for a 25-year-old woman it's about 25%, dropping to 10% at age 35. But how can you translate the MFR to how long it will wait to conceive (given regular unprotected sex, of course)?

To answer the question, Richie used the R language and the negative binomial distribution to calculate the probability of conceiving after 1 month, 2 months, 3 months etc. for women of various ages. (You can see the R code in Richie's blog post.) The results are summarized in the chart below:

So, if you're a 25-year old woman (see the red line above), it's more likely than not you'll have conceived by the third month of trying (about a 57% chance, in fact), and there's about a 95% chance of conceiving in the first year. At age 40 (blue), the chance of conception in the first year drops to around 50%. It's a much more challenging scenario for older women: at age 45, even after five years of trying it's still less than even odds of conception. This is what makes alternative means such as IVF a compelling option compared to the natural process.

For the complete details of the analysis, follow the link to Richie Cotton's blog below.

4D Pie Charts: How long does it take to get pregnant?

Thanks for noticing my post! I think it's a nice example of how a real world issue can be explained with just a few lines of R code and the right visualisation.

I should point out (as several commenters on my blog noticed) that the negative binomial distribution breaks down once you start looking at probabilities over several years, since the woman is getting older, and her MCR is changing. So don't leave it too late!

Also, the are loads of other factors that aren't taken into account like health problems, levels of smoking and drinking, obesity, stress and, of course, the fertility of the man.

Good luck to anyone else trying!

Posted by: Richie Cotton | July 02, 2012 at 02:30