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Rachel Gurevich

Are the Age-Related Fertility Statistics Out of Date? Maybe, But Modern Stats Aren't Much Better

By June 20, 2013

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There's an interesting article in The Atlantic online, "How Long Can You Wait to Have a Baby?" written by Jean Twenge. The author claims that the age-related fertility statistics we are often given are based on an outdated study of French birth records from 1670 to 1830. She says women are being pressured into having children at a younger age, when in fact more modern research shows fertility declines less drastically then once believed.

She writes that women are being told statistics based on a time "before electricity, antibiotics, or fertility treatment." She has a point that more modern studies should be done. However, to be frank, if we're considering natural fertility, you don't need electricity, antibiotics, or fertility treatment.

I admit I didn't look at the study myself yet, but I'm going to assume that they don't take into consideration the dead women's fertility rates. In fact, if anything, the older women who made the study are more likely healthier and stronger. (The whole survival of the fittest thing.) I suppose one could connect the antibiotics to certain sexually transmitted diseases which can lead to infertility, but even today, antibiotics aren't often given in time to prevent fertility damage.

Regarding the modern studies she cites, the studies were limited in number. The author admits this herself late in the article, mentioning that the more modern studies she cited include only 400 women collectively over the age of 35. I'm not sure I'd take a bet on my family based on study samples that small.

The article also completely fails to mention the increased risk of birth defects and miscarriage as male age rises. Autism and schizophrenia have been associated with older male fathers, and it is something to take into consideration.

However, what bugs me the most about her article was this paragraph (emphasis mine):

"David Dunson's analysis revealed that intercourse two days before ovulation resulted in pregnancy 29 percent of the time for 35-to-39-year-old women, compared with about 42 percent for 27-to-29-year-olds. So, by this measure, fertility falls by about a third from a woman's late 20s to her late 30s. However, a 35-to-39-year-old's fertility two days before ovulation was the same as a 19-to-26-year-old's fertility three days before ovulation: according to Dunson's data, older couples who time sex just one day better than younger ones will effectively eliminate the age difference."

Well, hold on a moment. Adjusting the day of sex wouldn't eliminate the age difference. This is a twisting of what the study actually found. Yes, it's true, in this study, a woman's chances of conception two days before ovulation ages 35 to 39 came out to be the same as a 19 to 26 year olds on three days before ovulation. But that's only because the 19 to 26 year olds are having sex an entire day earlier, decreasing their odds of conception.

In other words, older couples who time sex just one day better don't eliminate the age difference. The 19 to 26 year olds who have sex a day earlier decreased their odds of conception.

The actual data from the study for 19 to 26 year olds (with men the same age as the female) is as follows (with most fertile day underlined):

Day of Ovulation - About 0.1 probability of conception

One day before ovulation - About 0.3 probability of conception

Two days before ovulation - About 0.5 probability of conception

Three days before ovulation - About 0.3 probability of conception

The actual data from the study for 35 to 39 year olds (with men the same age as the female) is as follows (with most fertile day underlined):

Day of Ovulation - Less than 0.1 probability of conception

One day before ovulation - Slightly less than 0.2 probability of conception

Two days before ovulation - 0.29 probability of conception

Three days before ovulation - Slightly less than 0.2 probability of conception

The most likely date of conception for both the younger women and older women is two days before ovulation, and there is an obvious decrease in the odds for conception for the older women - 0.29 compared to 0.50.

It's worth mentioning that in the older women group, those with male partners five years older had even lower odds of conception, only 0.15 probability on their most fertile day. I'm surprised Twenge didn't mention this.

(You can read this study yourself here.)

Dunson himself concludes in his study, "Women's fertility begins to decline in the late 20s with substantial decreases by the late 30s." Twenge, however, apparently didn't like that conclusion.

Do I think women should be shamed and guilt tripped into having children before they are ready? Absolutely not. I've written this over and over again. Do we need new studies? Absolutely. Could the statistics we have be slightly slanted towards the more ominous statistics? That may be true, too.

But could Twenge, who obviously had a negative experience with regards to age-related-fertility shaming, be slanting towards the more optimistic studies? Even if she admits they're small studies? Even if she has to misrepresent Dunson's study just a tad to prove her point? Yep.

Women and men have a right to know about the effects of aging on fertility based on the research we have available now. And The Atlantic article, as nice as the premise is, isn't telling women and men everything they need to know.

More things you need to know:

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