Bayesian selection of optimal rules for timing intercourse to conceive by using calendar and mucus

Objective

To find optimal clinical rules that maximize the probability of conception while limiting the number of intercourse days required.

Design

Multicenter prospective study. Women were followed prospectively while they kept daily records of menstrual bleeding, intercourse, and mucus symptom characteristics. In some cycles, women sought to conceive, whereas in other cycles, they sought to avoid pregnancy.

Setting

Four centers providing services on fertility awareness.

Patient(s)

One hundred ninety-one healthy women using the Billings Ovulation Method. Women were invited to enroll by their instructors if they satisfied the entry criteria. We excluded cycles in which mucus was not recorded on a day with intercourse.

Intervention(s)

None.

Main Outcome Measure(s)

Clinically identified pregnancies. There were 161 clinically identified pregnancies in 2,536 menstrual cycles from 191 women.

Result(s)

Our approach relies on a statistical model that relates daily predictors, such as type of mucus symptom, to the day-specific probabilities of conception. By using Bayesian methods to search over a large set of possible clinical rules, focusing on rules based on calendar and mucus, we found that simple rules that are based on days within the midcycle calendar interval that also have the most fertile-type mucus symptom present have high utility.

Conclusion(s)

Couples can shorten their time to pregnancy efficiently by timing intercourse on days that the most fertile-type mucus symptom is observed at the vulva.

Key Words: Bayesian analysis, cervical mucus, conception, decision theory, fertility awareness, natural family planning, time to pregnancy