Context: Basal follicle stimulating hormone (FSH) is a marker for ovarian reserve.

Objectives: To investigate the predictive value of basal FSH on spontaneous ongoing pregnancy in subfertile ovulatory women.

Design: Prospective cohort study.

Setting: Nineteen fertility centers in The Netherlands.

Participants: Subfertile ovulatory women without two-sided tubal pathology and in whom the man had normal sperm parameters (Total motile count 3x10⁶).

Interventions: Fertility work-up, including a basal FSH measurement on cycle day 3.

Main outcome measures: Spontaneous ongoing pregnancy.

Results: We included 3,519 consecutive couples of which 562 (16%) had a spontaneous ongoing pregnancy within one year. Basal FSH levels of 8 IU/L or higher were associated with a decreased probability of spontaneous ongoing pregnancy (FR 0.93 per IU/L [95% CI 0.87 to 0.98]). In a multivariable analysis, female age (FR 0.97 per year, 95% CI 0.95 to 0.99), cycle length (FR 0.96 per day, 95% CI 0.93 to 1.0) and FSH levels 8 IU/L (FR 0.93 per IU/L, 95% CI 0.87 to 0.99) were strong negative predictors for spontaneous ongoing pregnancy.

Addition of FSH to a prediction model based on female age, duration of subfertility, previous pregnancy, referral status and semen analysis changed the probability to conceive spontaneously from 30% to < 30% in 97 out of 3,219 couples (3.0%).

Conclusions: In ovulatory women, a basal FSH level of 8 IU/L or higher is associated with decreasing fecundity, independent of female age and cycle length. As the number of couples in whom the FSH level alters management decisions is low, we do not recommend routine testing of basal FSH in subfertile couples.