Chances of Getting Pregnant: Math Estimates the Answers

20% 25% or 30% Chances of Conceiving?

Months for Pregnancy Calculator via Spreadsheet : image by Mike DeHaan

Months for Pregnancy Calculator via Spreadsheet : image by Mike DeHaan

The bright blue columns show P(n), the probability that you would conceive exactly in that cycle. To the right is the cumulative probability that you would conceive no later than that month.

The green highlight value shows the “expected” value where at least half the women would have conceived.

The “deviation” value shows the month by which about 68% of women would have gotten pregnant under each of the probability values.

Notice that the monthly probabilities become smaller, and the cumulative ones increase, as the months go by. That’s because each monthly probability represents the chance you would become pregnant precisely in that one month.

However, a statistician would be unlikely to reject the hypothesis that you could conceive a child, doing what you’re doing, until the cumulative probability has gone past 95%. That’s somewhere between 9 and 14 months on this chart.

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Math Cannot Guarantee Conception Predictions

When researchers report the monthly success rates for getting pregnant, the results may not fit the geometric probability distribution perfectly.

Each person’s chance of conceiving might change from month to month, depending on their behaviour or environment.

Your fertility monitor might be a day ahead or behind; perhaps your schedule changes unavoidably. Many factors may influence your fertility from one month to another; or you might have a very steady chance of getting pregnant each time – it’s better to rely on your doctor’s perspective than in math in this case.

So What are the Chances of Getting Pregnant?

Research indicates that healthy, sexually active couples of childbearing age have a nearly 40% chance of conception in their first monthly cycle of ‘trying.’ Assuming that they do nothing to alter their chances of getting pregnant from month to month, the math of the geometric probability distribution shows a very high cumulative probability of conception within the first year.


C. Gnoth, D. Godehardt, E. Godehardt, P. Frank‐Herrmann and G. Freundl. Time to pregnancy: results of the German prospective study and impact on the management of infertility. (2003). Accessed May 3, 2013.

Getting Pregnant. How Long Will it Take Me to Get Pregnant? (2013). Accessed May 3, 2013.

Kelmon, Jessica. How long it takes to get pregnant. (2013). Baby Center. Accessed May 3, 2013.

AI Access.Geometric Distribution. Accessed May 3, 2013.

Weisstein, Eric WExpectation Value and Geometric Distribution. From MathWorld–A Wolfram Web Resource. Accessed May 3, 2013. 


The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment. If you need medical advice, talk with a doctor.

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