November 6, 2015

Double Header – A Miscarriage Estimator

On Monday I released the Passive Voice Detector, today I am releasing my second app of the week, the Miscarriage Estimator.

After the success of my Labor Predictor, I started thinking about a Miscarriage calculator as a natural follow on idea. I like math, so diving into the statistics generally make me feel better about things that are beyond my control. I’ve also been around long enough to know that’s not necessarily true for everyone, or even most people. In my passive observations, there didn’t seem to be enough interest in such an app to make creating it worth while. I tabled the idea.

Then a dear friend suffered a loss.

As devastating as the loss itself can be, uncertainty can make it so much worse. On bad days my friend worried she would never get pregnant again, or if she did she would continue to miscarry. After reading on the internet that she was at five times greater risk for recurrent miscarriages, she was understandably distraught. So we talked about where the number came from, and that while her risk may have increased, it was still nearly 20 times more likely she had just gotten unlucky than she had an underlying condition. (Explanation below if you’re interested.) Throughout the subsequent months and her following pregnancy we sent many long nights emailing each other back and forth, breaking down the scary statistics and rephrasing them in a more positive way.

That is my goal with the Miscarriage Estimator, to take a scary statistic and make it less so. 20% of pregnancies may result in miscarriage also means 80% of pregnancies are carried to term. Knowing doesn’t have to be scary.

Examining the odds of recurrent miscarriage:

The intuition: the rate of miscarriage is generally assumed to be 20%. The rate of recurrent miscarriage is about 1%. That means of a group of 100 newly pregnant women we might expect 99 to have no underlying issues and 1 to have a as yet unknown recurring miscarriage condition. Of the 99 women without issue, we might expect 20 will be unlucky and miscarry by just random chance. The one with the recurrent miscarriage will also likely miscarry, for a total of 21 miscarriages out of 100. The odds of being the one with the underlying recurrent miscarriage condition increased from 1 in 100 to 1 in 21. While five times higher, it’s still 20 times more likely for a woman who had a miscarriage to not be the one with an underlying recurrent miscarriage condition.

Mathematically:
p(recurrent_miscarriage | single_miscarriage) = p(single_miscarriage | recurrent miscarriage) * p(recurrent_miscarriage) / p(single_miscarriage) By Bayes Rule
p(recurrent_miscarriage | single_miscarriage) = 100% * 1% / 20%
p(recurrent_miscarriage | single_miscarriage) = 5%

p(not_recurrent_miscarriage | single_miscarriage) = p(single_miscarriage | not_recurrent miscarriage) * p(not_recurrent_miscarriage) / p(single_miscarriage) By Bayes Rule again
p(not_recurrent_miscarriage | single_miscarriage) = 20% * 99% / 20%
p(not_recurrent_miscarriage | single_miscarriage) = 99%

Thus the probability of not having a recurrent miscarriage condition is 20 times greater (99% vs 5%) than the probability of having it.

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