What is the significance of antral follicles on Transvaginal Ultrasound (TVUSG) in evaluating ovarian reserve and fertility potential in a woman of reproductive age?

Antral Follicle Count on Transvaginal Ultrasound: Assessment of Ovarian Reserve and Fertility Potential

Antral follicle count (AFC) on transvaginal ultrasound is a critical marker of ovarian reserve that directly predicts fertility potential, with fewer than 5 antral follicles indicating diminished ovarian reserve and requiring immediate hormonal assessment and reproductive endocrinology referral. 1

Diagnostic Thresholds and Clinical Significance

Normal vs. Diminished Reserve

• When ovarian volume is <3 cm³ combined with <5 antral follicles, this indicates diminished ovarian reserve—a pathologic finding regardless of cycle timing that warrants urgent evaluation. 1
• Normal ovarian reserve is characterized by 10-23 antral follicles (2-10 mm diameter) per ovary, with ovarian volume >3 cm³. 1
• The right ovary typically contains more antral follicles than the left ovary in women with both normal reserve and polycystic ovarian morphology. 2

Polycystic Ovarian Morphology

• Polycystic ovarian morphology requires ≥25 follicles (2-9 mm diameter) in at least one ovary when using an 8 MHz or higher frequency transvaginal transducer, or ovarian volume >10 mL. 3
• The older threshold of 20 follicles has been superseded by the 25-follicle criterion established by the Androgen Excess and Polycystic Ovary Syndrome Society. 3

Technical Considerations for Accurate Assessment

Follicle Size Matters

• Small antral follicles (2-5 mm) are the most clinically significant population—their count correlates more strongly with ovarian response to stimulation than total AFC. 4, 5
• The number of 2-6 mm follicles declines with age and correlates with all endocrine ovarian reserve tests, representing the functional ovarian reserve. 5
• Larger follicles (6-10 mm) remain relatively constant with age and have limited predictive value except for basal inhibin B levels. 5
• A high ratio of small (2-5 mm) to total antral follicles predicts better ovarian response to controlled ovarian stimulation. 4

Optimal Imaging Protocol

• Use a transvaginal transducer with ≥8 MHz frequency for accurate follicle detection and counting. 3, 1
• Perform AFC assessment in the early follicular phase (cycle days 2-5) for optimal standardization, though AFC can be measured throughout the cycle. 1
• Document exact ovarian volume in cubic centimeters for both ovaries, as volume <3 cm³ combined with low follicle count confirms diminished reserve. 1

Integration with Hormonal Assessment

AMH as the Gold Standard

• Anti-Müllerian hormone (AMH) is the most reliable marker of ovarian reserve and can be measured on any cycle day, making it more convenient than AFC. 1, 6
• AMH demonstrates significantly less inter-cycle and intra-cycle variability than AFC (ICC 0.89 vs 0.71 for inter-cycle; 0.87 vs 0.69 for intra-cycle), making it more robust for clinical decision-making. 7
• However, AMH fluctuates throughout the menstrual cycle particularly in women <25 years, requiring cautious interpretation in younger patients. 3, 1

Complementary Hormonal Testing

• Measure FSH and estradiol on cycle days 2-3 when menstrual dysfunction suggests premature ovarian insufficiency or when fertility assessment is desired. 3, 1
• Day 21 progesterone (midluteal phase of a 28-day cycle) confirms ovulation. 1
• TSH screening rules out thyroid disorders affecting fertility. 1

Clinical Action Based on AFC Findings

Diminished Ovarian Reserve (<5 Follicles)

• Immediately refer to reproductive endocrinology for fertility preservation counseling, with oocyte cryopreservation as the primary option for postpubertal patients. 8
• Initiate sex steroid replacement therapy to prevent bone loss, cardiovascular disease, and sexual dysfunction from estrogen deprivation. 8
• Obtain AMH level to confirm diminished reserve (AMH <35 pmol/L or <5 ng/mL supports the diagnosis). 9
• Consider hysterosalpingography to assess tubal patency if actively pursuing pregnancy. 1

Normal Reserve (10-23 Follicles)

• Proceed with standard fertility evaluation including tubal patency assessment and semen analysis. 1
• Counsel regarding age-related decline in fertility and optimal timing for conception attempts. 6

Polycystic Ovarian Morphology (≥25 Follicles)

• Evaluate for clinical and biochemical hyperandrogenism to diagnose polycystic ovary syndrome. 3
• Test for glucose intolerance, insulin resistance, and lipid abnormalities. 1
• Do not pursue PCOS workup when ovarian volumes are normal (<10 mL) and clinical hyperandrogenism is absent, as this leads to misdiagnosis. 9

Critical Pitfalls to Avoid

• Never dismiss significant ovarian asymmetry (e.g., one ovary <3 cm³ while the other is normal) as benign variation—this warrants investigation for unilateral pathology or systemic ovarian dysfunction. 9
• Do not assume a single follicle with low ovarian volume represents normal function during a particular cycle phase—this is pathologic regardless of timing. 1
• Transabdominal ultrasound is unreliable for accurate follicle counting; use transvaginal approach or MRI if transvaginal is not feasible. 3
• If ovaries cannot be adequately visualized transvaginally (e.g., in obese patients), obtain MRI pelvis without IV contrast for accurate volume and follicle assessment. 3, 1

Special Populations

Cancer Survivors

• Survivors treated with alkylating agents (cyclophosphamide, procarbazine) or pelvic radiotherapy have increased risk of premature ovarian insufficiency. 3
• Annual surveillance with menstrual history and physical examination is recommended, with FSH/estradiol testing for menstrual dysfunction. 3
• No high-quality studies exist validating AFC or AMH for predicting premature ovarian insufficiency in cancer survivors, though expert opinion supports their use for assessing ovarian reserve. 3

Prepubertal and Peripubertal Patients

• Monitor growth and pubertal development annually, with increasing frequency based on progression. 3
• Perform laboratory evaluation (FSH, estradiol) for girls ≥11 years who fail to initiate or progress through puberty normally. 3
• Ovarian tissue cryopreservation is the only fertility preservation option for prepubertal patients with diminished reserve. 8