Figure 1. The solution of the Faddy–Gosden differential equation for the primordial follicle population from birth to menopause. The primordial follicle population at birth is ∼701 000 ( A ), and at menopause is ∼1000 at 50.4 years ( C ), with an accelerated decline occurring at ∼25 000 remaining primordial follicles ( B ). From: Ovarian reserve and reproductive age may be determined from measurement of ovarian volume by transvaginal sonography Hum Reprod. 2004;19(7):1612-1617. doi:10.1093/humrep/deh285 Hum Reprod | European Society of Human Reproduction and Embryology

Figure 2. Upper panel: distribution of ages at menopause in 529 women followed prospectively. Mean age at menopause 50.4 years, SD 3.9 years (Treloar, 1980). Y is the mean age at menopause (50.4) years. X is 42.8 years; the 95% lower confidence limit for age at menopause (50.4 – 1.96 × 3.9) years. Z is 58.0 years; the 95% upper confidence limit for age at menopause (50.4 ± 1.96 × 3.9) years. Here 1.96 is the Student’s t critical point for a 95% CI for a normal distribution. Lower panel: combining primordial follicle population decline and age at menopause for early (X), average (Y) and late menopause (Z). Points X, Y and Z are derived as in the upper panel, and all occur at a primordial follicle population of 1000 (best estimate of number of remaining follicles at menopause) represented by the horizontal dotted line. The lines terminating at X and Z are the solutions for the Faddy–Gosden equation where 1000 follicles remain at 42.8 and 58.0 years, respectively. For example, a 35‐year‐old woman is expected to have a primordial follicle population of between 10 000 (LP) and 90 000 (UP). This corresponds to a reproductive age of between 27.4 (LA) and 42.6 (UA) years. LA = lower reproductive age; UA = upper reproductive age; LP = lower primordial follicle population; UP = upper primordial follicle population. From: Ovarian reserve and reproductive age may be determined from measurement of ovarian volume by transvaginal sonography Hum Reprod. 2004;19(7):1612-1617. doi:10.1093/humrep/deh285 Hum Reprod | European Society of Human Reproduction and Embryology

Figure 3. Mean ovarian volume and 95% upper confidence limit related to age for ages 25–51. Redrawn from Pavlik et al. (2000). The number of observations are as follows: 25–29 years, 444; 30–39 years, 3259; 40–49 years, 9963; and 50–51 years, 4300. From: Ovarian reserve and reproductive age may be determined from measurement of ovarian volume by transvaginal sonography Hum Reprod. 2004;19(7):1612-1617. doi:10.1093/humrep/deh285 Hum Reprod | European Society of Human Reproduction and Embryology

Figure 4. Upper panel: correlation between log mean primordial follicle population from our solution of the Faddy–Gosden equation (Figure 1) and mean ovarian volume for each chronological age from 25 to 51 years (from Pavlik et al. , 2000) (Figure 3). Using a least squares fit: log ₁₀y  = 0.53 x  + 1.3, with r  = 0.97. Example data points are as follows: age 48 years, ovarian volume 4.0 ml, primordial follicle population 2300; age 28 years, ovarian volume 6.7 ml, primordial follicle population 85 100. Lower panel: correlation between (log) primordial follicle population for late menopause women (Figure 2) and 95% upper confidence limit for ovarian volume (Pavlik et al ., 2000) (Figure 3) for ages 25–58 years. Using a least squares fit: log ₁₀y  = 0.19 x  + 2.5, with r  = 0.96. Example data points are as follows: age 51 years, ovarian volume 6.0 ml, primordial follicle population 7300; age 41 years, ovarian volume 12.0 ml, primordial follicle population 43 900. From: Ovarian reserve and reproductive age may be determined from measurement of ovarian volume by transvaginal sonography Hum Reprod. 2004;19(7):1612-1617. doi:10.1093/humrep/deh285 Hum Reprod | European Society of Human Reproduction and Embryology

Figure 5. Reproductive age (years) versus ovarian volume (ml) for 40‐year‐old women. Point A (32.4 years, 12.8 ml), i.e. 40 – (1.96 × 3.9) years, the upper limit on 95% CI for ovarian volume. Point B (40 years, 5.9 ml), i.e. chronological age, average ovarian volume for age 40. Point C (50.4 years, 2.2 ml), i.e. average age of menopause, average ovarian volume for post‐menopausal women. Example calculations are shown for two individuals aged 40 years with mean ovarian volumes of 9 and 5 ml giving reproductive ages of 36 and 42 years, respectively. Predicted age at menopause is [average age at menopause plus (chronological age minus reproductive age)]. For these examples, the predicted age at menopause is 54 years and 48 years, respectively. From: Ovarian reserve and reproductive age may be determined from measurement of ovarian volume by transvaginal sonography Hum Reprod. 2004;19(7):1612-1617. doi:10.1093/humrep/deh285 Hum Reprod | European Society of Human Reproduction and Embryology

Figure 6. Reproductive age (years) versus ovarian volume (ml) for women of 25–45 years at 5 yearly intervals. Line A, women of chronological age 25 years; line B, women of chronological age 30 years; line C, women of chronological age 35 years; line D, women of chronological age 40 years; line E, women of chronological age 45 years. Number of years remaining before the menopause is 50–51 years minus reproductive age. From: Ovarian reserve and reproductive age may be determined from measurement of ovarian volume by transvaginal sonography Hum Reprod. 2004;19(7):1612-1617. doi:10.1093/humrep/deh285 Hum Reprod | European Society of Human Reproduction and Embryology