Changes with age in the level and duration of fertility in the menstrual cycle.
Level 3 - non-randomized controlled study
Prospective cohort study analyzing day-specific pregnancy probabilities across age groups
PubMed 11980771 · doi:10.1093/humrep/17.5.1399
What was done
Prospective cohort study of 782 healthy couples using natural family planning methods who provided daily intercourse records across 5,860 menstrual cycles. Ovulation timing was determined by basal body temperature measurements. Investigators calculated day-specific probabilities of pregnancy relative to ovulation and measured the length of the fertile window across female and male age groups, uncoupling biological aging effects from intercourse frequency.
What was found
Almost all pregnancies occurred within a 6-day fertile window, and the length of this window did not shorten with advancing age in men or women. Day-specific pregnancy probabilities began declining for women in their late 20s, with probabilities roughly twice as high in women aged 19–26 years compared to those aged 35–39 years. After adjusting for female age, male fertility was also significantly reduced for men aged >35 years.
Why it matters
By controlling for the timing and frequency of intercourse, this study confirms that age-related fertility decline is driven by biological factors in both women (starting in the late 20s) and men (after age 35), rather than changes in sexual behavior or fertile window duration.
Limits
Ovulation timing was identified via basal body temperature rather than ultrasound or serum hormone monitoring. The sample was limited to couples utilizing natural family planning, which may select for specific demographic, behavioral, or reproductive characteristics. Specific numerical probabilities and confidence intervals were not reported in the abstract.
Cited by
- contradicts In women who previously had a child and are conceiving with the same partner, monthly fecundability remains between 18% and 20% up until age 37 before dropping.