Fertility probability and age

4 min read

Men and women alike often have an astonishing misapprehension of fertility probability.  

Part of the problem is our society’s anti-scientific cult of optimism (e.g., “I’m baselessly certain your serious problem will magically turn out fine. Shut up and stop making rational plans based on empirical data.”).  Part of the problem is also a curious lack of well-presented, easily-digestible data. 

Charts of Monthly Conception Probability by Age are readily available online.  But, that raw data is not very useful.  One really needs to understand the Probability of Healthy Birth by Age. 

To figure that out, you’d have to do the following calculations:

  • 1 – Monthly Conception Probability by Age = Monthly Failure Rate by Age
  • 1 – Monthly Failure Rate by Age ^ # of Months Tried = Theoretical Probability of Conception by Age
  • Multiply by the Probability of Male Infertility
  • Multiply by the Probability of Female Physiological Fertility Barriers
  • Multiply by the Probability of Miscarriage by Age
  • Multiply by the Probability of Chromosomal Abnormalities by Age

The vastly more useful result:
Conclusion #1:  Fertility begins declining at a younger age than most people think. 
Conclusion #2:  Rejecting one woman in favor of a younger one for the sole reason of presumed fertility is a risky wager.  Test this out yourself in our handy calculator:

Fertility probability calculator
Woman A
Woman B
Age today
Please input an integer from 20 to 48
For Woman B, please input an integer from 20 to 48 that is equal to or less than your input for Woman A
Probability of live healthy birth within 33 months from today (24 monthly attempts with a man of unknown fertility status) 30% 71%
Probability that it's a MISTAKE to reject Woman A in favor of Woman B purely due to age-based fertility assumptions. 60%

Yes, you increase your odds with a younger woman.  But the probability you’ve made a mistake is often quite high.  Why is it so often a bad wager to reject the older woman for the younger one?

  1. Infertility of 30-something women is higher than most people think. You’re not trading up in probabilities as much as you might think.  (Conclusion #1 above)
  2. Infertility rate of men is not zero.  If the man is infertile, then the woman’s age never mattered.  (Therefore, men should test themselves BEFORE they ever form any fertility-related age preferences for their mate search.)  
  3. Though far lower than most people think, the fertility rate of 40-something women is not zero. Note that Trump had only a 5-20% chance of winning the election, and yet he won.  If there’s a 20% chance of rain in the forecast, you probably bring a jacket.

Other points that surprise many people:

  • Any birth after age 45-ish probably used a donor egg (or the woman’s own previously frozen eggs). It shouldn’t be surprising that new parents don’t often volunteer their use of donor eggs.  Also, for every miraculous older woman birth you read about in US Weekly, there are hundreds of cases you don’t hear about where it wasn’t possible.
  • Sterility often occurs 5-10 years before menopause. Menstruation in a 40-something doesn’t necessarily mean she can conceive.
  • IVF doesn’t change the odds unless you use a donor egg or donor sperm
  • Male infertility doesn’t appear to be age-related (though chromosomal abnormalities may go up with older fathers)
  • For ~15% of unable-to-conceive couples, the cause is not age.  The man’s fertility is the culprit, and/or a physiological problem in the woman’s body.  
  • Overcoming fertility challenges is prohibitively expensive for most people. It’s all out-of-pocket.  It’s not unheard of to have a $300,000 baby.  New parents don’t tend to volunteer the true all-in costs:
    • Fertility testing: ~$1k
    • Freezing eggs: ~$12k
    • Insemination with donor sperm: ~$2k
    • IVF and implantation
      • own previously-frozen egg: ~$15k
      • donor egg, own womb: ~$30k
      • donor egg, surrogate womb: ~$100k

This is yet another case where math literacy concretely impacts people’s lives.  Innumeracy of the general population combines with poor data presentation by experts to cause real harm.  People routinely cheerlead 30- and 40-something single women with false platitudes that they have plenty of time to find a mate and have biological children.  People also routinely hand-waive away women’s known fertility test results with non-representative and irrelevant hearsay accounts of middle-aged births.  Logic and data are sacrificed to preserve the unaffected party’s comfortable emotional disengagement.  Once well-meaning people let go of data-blind optimism regarding biology, they often persist with the related, second-order “you can always adopt” platitude – though that, too, often proves false (for legal and financial reasons) for would-be mothers.

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