Objective: To determine the efficacy of using natural family planning (NFP) methods to avoid unintended pregnancy among women of perimenopause age (i.e., age 40-55 years).

Design: A secondary analysis of subset data from two prospective observational cohort studies.

Setting: A university based in-person and online NFP service program.

Participants: One hundred and sixty couples who used either a website or an in-person NFP service to learn how to avoid pregnancy from January 2001 to November 2012.

Methods: A prospective 12-month effectiveness study among 160 women (between ages 40-55) who used NFP to avoid pregnancy. The women used either a hormonal fertility monitor, cervical mucus monitoring, or both to estimate the fertile phase of their menstrual cycles. Survival analysis was used to determine the pregnancy rate over 12 months of use.

Results: There were a total of five unintended pregnancies among the participants. The typical use pregnancy rate was six per 100 women over 12 months. The monitor alone participants (n = 35) had a 12-month pregnancy rate of three, the participants (n = 73) who used mucus alone had a pregnancy rate of four, and the participants (n = 42) who used the fertility monitor plus mucus had a pregnancy rate of six.

Conclusion: Natural family planning methods can be effective for older women to avoid an unintended pregnancy with correct use and adequate instructions. The pregnancy rate most likely was affected by diminished fertility and motivation to limit family size.