What is the initial approach for a female of reproductive age with a single follicle present on transvaginal ultrasound (USG)?

Single Follicle on Transvaginal Ultrasound: Initial Approach

A single follicle on transvaginal ultrasound in a reproductive-age woman suggests diminished ovarian reserve and requires immediate hormonal assessment with AMH, FSH, and LH on cycle day 2-3, along with measurement of ovarian volume to confirm this concerning finding. 1

Interpretation of the Ultrasound Finding

• When ovarian volume is <3 cm³ and <5 antral follicles are present, this indicates diminished ovarian reserve, which is a significant concern for fertility potential 1, 2
• A single follicle falls well below the threshold of 5 antral follicles, making diminished ovarian reserve the primary diagnostic consideration 1
• This finding is distinctly different from polycystic ovarian morphology, which requires ≥20 follicles (2-9mm diameter) per ovary or ovarian volume >10 mL 1, 3

Immediate Hormonal Assessment Required

• Measure anti-Müllerian hormone (AMH) regardless of cycle day, as this is the most reliable marker of ovarian reserve 2
• Obtain FSH and LH on cycle day 2-3 to assess ovarian reserve and pituitary function 2
• Measure midluteal phase progesterone (day 21 of a 28-day cycle) to confirm whether ovulation is occurring 2
• Check thyroid-stimulating hormone (TSH) to rule out thyroid disorders affecting fertility 2

Complete the Ultrasound Assessment

• Document the exact ovarian volume in cubic centimeters for both ovaries, as volume <3 cm³ combined with low follicle count confirms diminished reserve 1, 2
• Ensure the ultrasound was performed with an 8 MHz or higher frequency transvaginal transducer for accurate follicle detection 1
• Consider repeat ultrasound in the early follicular phase (cycle days 2-5) if the initial scan was performed at a suboptimal time in the cycle 1, 4

Additional Diagnostic Workup

• Obtain a complete blood count (CBC) to identify anemia or hematologic abnormalities that could impact fertility 2
• Screen for infectious diseases including chlamydia, gonorrhea, syphilis, and HIV 2
• Consider hysterosalpingography to assess tubal patency if the patient is actively pursuing pregnancy 1

Critical Clinical Pitfalls to Avoid

• Do not assume this represents normal ovarian function during a particular cycle phase—a single follicle with low ovarian volume is pathologic regardless of cycle timing 1
• Do not delay hormonal testing while waiting for another cycle, as time is critical in diminished ovarian reserve 2
• If ovaries cannot be adequately visualized by transvaginal ultrasound, obtain MRI pelvis without IV contrast to accurately assess ovarian volume and follicle counts 1
• Do not confuse this finding with polycystic ovarian morphology, which shows the opposite pattern (multiple follicles) 1, 3

Prognostic Implications

• Diminished ovarian reserve significantly impacts fertility potential and requires prompt referral to reproductive endocrinology 2
• The combination of low antral follicle count and small ovarian volume predicts poor response to ovulation induction 1, 4
• Age-related decline in follicle count accelerates after age 37 years, with a mean yearly decline of 11.7% compared to 4.8% before age 37 4