How does fertility change around ovulation?

Couples seeking to avoid or achieve pregnancy often wonder how likely pregnancy is under a variety of circumstances. Understanding how to risk of pregnancy changes is a key part of natural family planning. Even within the "fertile window" which NFP identifies, the chances of pregnancy vary.

In 1995, a paper, a study of 221 healthy women trying to have children found that, of the 192 pregnancies which occurred, all pregnancies had been conceived in a seven day window around ovulation. 

In the study, ovulation dates were calculated retrospectively. Each morning, participating women collected a urine sample, which researchers then analyzed for the hormone shifts which indicate ovulation. Women also recorded when they had intercourse. Obviously, it can be difficult to attribute any one instance of intercourse to conception, but the researchers found that during cycles where conception occurred, couples had always had sex within a window of 6 days prior to ovulation to 1 day afterwards.  Similarly, in cycles where the couples did not have sex within that 7 day period, no pregnancies occurred. To obtain the graph above, statistical models were applied to the original data. Read the original findings here.

What does this mean for natural family planning?

1. Sperm can survive a surprising 6 days in the female reproductive tract, dramatically extending the period of time that intercourse can lead to pregnancy.
2. Couples who are trying to achieve a pregnancy should focus on the week before ovulation, and especially a day or two before ovulation.
3. Following ovulation, fertility drops quickly.