Lies, damned lies and statistics

Going right back to the beginning of the pregnancy journey, there was a fascinating article on BBC News yesterday based on a new book (The Impatient Woman’s Guide to Getting Pregnant) about the sources for statistics used about conception and fertility rates.

A few key excerpts:

“Take this often-cited statistic: one out of three women over the age of 35 will not have conceived after a year of trying.

The data on which that statistic is based is from 1700s France. They put together all these church birth records and then came up with these statistics about how likely it was [someone would] get pregnant after certain ages.”

“A graph based on 17th-19thC birth records still features in the “Figures and Tables” section of the 2013 Nice fertility guideline”.

It amazes me even if it doesn’t entirely surprise me that some of the data being used to advise us is completely out of date. We worry about having children in our early thirties rather than late thirties when in reality there is no “cliff” when you turn thirty five that, as you blow out the candles on your cake, “poof” goes your fertility as well.

As the article mentions, if you actually use a more up-to-date study (Dunson et al, 2004) you find that the story is a lot more rosy for those in their late thirties with conception rates above 80% within a year. For those that are interested, I have included some more on the findings from the study at the bottom.

Because of the ethics involved in conception, pregnancy and birth, the ability to run true experiments where you can control factors is limited, the science becomes more like social science where you can’t control much if anything and just hope you can identify the main factors contributing to an effect (something the Chancellor will be wringing his hands about in the case of austerity economics!).

I guess the moral of the story is one drummed into me during GCSE History – know the reliability of your sources. If a statistic is making you change your behaviour then it is worth knowing where it came from, and judge for yourself its relevance. And if NICE are guilty of dubiously relevant statistics then anyone could be at it.

Trust no one Mr Mulder!

More from the Dunson et al 2004 study

The graphs show the probability of falling pregnant for each age group cumulatively against the number of menstrual cycles they have been trying for. The difference between the first and the second graphs is that in the first, couples were having sex twice a week and in the second once (interestingly there is no graph for three times a week because they found no statistical difference between two and three times).

“The proportion of women failing to conceive within 12 cycles (thus meeting the criterion for clinical infertility) ranges from 8% for 19- to 26-year-olds to 13–14% for 27- to 34-year-olds, to 18% for 35- to 39-year-olds. If frequency of intercourse is reduced to once per week, the rates of infertility increase substantially to 15%, 22–24%, and 29% for women aged 19–26, 27–34, and 35–39 years, respectively”.

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