Ovarian Ultrasound: Normal Values, Follicle Counts, and PCOS

Pelvic ultrasound is one of the most requested studies in outpatient gynecology, and the ovaries are almost always part of the evaluation. Whether you're documenting a routine pelvic exam, evaluating for PCOS, or characterizing a complex adnexal mass, knowing the normal values and measurement technique cold makes a real difference in the quality of your study.

Normal Ovarian Size

Ovarian size varies considerably with age, hormonal status, and phase of the menstrual cycle. The following represent generally accepted reference ranges:

Reproductive-Age Women (18–40)

Measurement	Normal Range
Length (craniocaudal)	2.5–5.0 cm
Width	1.5–3.0 cm
Height (AP)	1.5–3.0 cm
Volume	6–10 mL (typical) / ≤20 mL (upper limit)

Volume is calculated using the prolate ellipsoid formula:

Volume = Length × Width × Height × 0.523

Most practices report ovarian volume rather than three separate dimensions for clarity and reproducibility.

Postmenopausal Women

Ovaries become progressively smaller and more difficult to identify after menopause:

Years Postmenopause	Expected Volume
1–5 years	3–6 mL
>5 years	1–3 mL
>10 years	Often <2 mL, may be unidentifiable

A postmenopausal ovary >8 mL is abnormal and warrants further workup. An ovary that cannot be identified in a postmenopausal woman is not always pathological — document that it was not visualized rather than assuming it's absent.

Prepubertal Girls

Ovaries are small and oval, typically <1 mL before puberty. Small follicles (2–9 mm) are normal in prepubertal ovaries and should not be reported as pathological.

How to Measure the Ovary

Transabdominal vs. Transvaginal

Transvaginal ultrasound (TVUS) is the gold standard for ovarian evaluation. The higher frequency (5–9 MHz) provides superior resolution for follicle counting, small lesions, and ovarian morphology. Use transvaginal whenever possible in adult patients with clinical indication.

Transabdominal ultrasound (TAUS) with a full bladder is used in patients who decline transvaginal imaging, in adolescents, or when a large mass extends beyond the pelvis. Resolution is lower, and small follicles may not be identifiable.

Finding the Ovary

The ovary is typically located:

• Lateral to the uterus, medial to the iliac vessels
• Posterior to the broad ligament
• In anteverted uteri: superior-lateral to the uterine fundus
• In retroverted uteri: the ovaries may be posterior or in the cul-de-sac

The iliac vessels are your landmark. Identify the internal iliac artery and vein — the ovary is almost always just medial to them. Apply gentle pressure with the probe to separate it from adjacent bowel.

Measurement Technique

Obtain three orthogonal dimensions:

1. Length — the longest craniocaudal dimension
2. Width — the widest transverse dimension, on the same image as length
3. Height — the AP dimension, measured on a perpendicular plane

Always measure the ovarian parenchyma only — include follicles as part of the ovary, but do not include adjacent free fluid or a dominant cyst that appears separate from the ovarian tissue.

Calculate and report volume using: L × W × H × 0.523

Follicle Measurement and Antral Follicle Count

Normal Follicle Development

During a normal cycle:

• Days 1–5: Multiple small antral follicles (2–10 mm), usually bilateral
• Days 5–10: One follicle dominates (dominant follicle), grows 1–2 mm/day
• Days 12–14: Dominant follicle reaches 18–24 mm (preovulatory)
• Ovulation: Follicle disappears or collapses; free fluid may appear in the cul-de-sac
• Luteal phase: Corpus luteum forms — may appear as a thick-walled cystic structure with ring of vascularity on Doppler ("ring of fire")

Measuring Follicles

• Measure in two perpendicular planes and average (or report both dimensions)
• Small follicles (2–9 mm) are measured on the same image by sweeping through the ovary
• For fertility monitoring, the dominant follicle is the key measurement

Antral Follicle Count (AFC)

AFC is the total number of small follicles (2–10 mm) across both ovaries, counted during the early follicular phase (cycle days 2–5). It's a primary marker of ovarian reserve.

AFC (both ovaries combined)	Interpretation
≥16	Normal / good reserve
7–15	Average reserve
3–6	Low reserve, may respond poorly to stimulation
<3	Very low reserve (poor prognosis for fertility)

Technique for AFC:

• Use transvaginal probe at highest frequency
• Sweep from medial to lateral edge of each ovary in real time
• Count every follicle measuring 2–10 mm — do not count dominant follicles or cysts
• Some labs use software-assisted counting (SonoAVC) for reproducibility

Polycystic Ovarian Morphology (PCOM)

Revised 2023 Rotterdam Criteria — Ultrasound Component

The most recent international PCOS guidelines (2023 International Evidence-based Guideline) updated the sonographic threshold for polycystic ovarian morphology:

Either criterion qualifies:

• Follicle number per ovary (FNPO) ≥20 (on transvaginal ultrasound using modern high-frequency probes)
• Ovarian volume ≥10 mL on either side (excluding dominant follicle or cysts)

Important: The older threshold of ≥12 follicles per ovary (from the 2003 Rotterdam criteria) is outdated for modern high-frequency probes and should no longer be used.

PCOM on ultrasound alone does not diagnose PCOS. PCOS requires at least 2 of 3 criteria:

1. Irregular or absent ovulation
2. Clinical or biochemical hyperandrogenism
3. Polycystic ovarian morphology on ultrasound

Note for adolescents: PCOM on ultrasound should not be used to diagnose PCOS in patients within 8 years of menarche — enlarged, multi-follicular ovaries are common in adolescence and not specific to PCOS.

Classic Sonographic Appearance of PCOS

• Multiple small follicles (typically 2–9 mm), often arranged peripherally — the "string of pearls" sign
• Increased central echogenic stroma
• Ovarian volume often ≥10 mL
• Ovaries may appear bilaterally enlarged and rounded

The string of pearls pattern is visually striking but not required for PCOM. What matters is the follicle count and/or volume meeting threshold.

Common Adnexal Findings and What to Document

Simple Cysts

• Anechoic, thin walls, posterior acoustic enhancement, no internal flow
• Reproductive age: Follicular cysts (≤3 cm are normal), corpus luteum cysts (≤5 cm, thick wall, ring of fire flow)
• Postmenopausal: Simple cysts ≤1 cm are almost always benign; 1–7 cm requires follow-up; >7 cm warrants further evaluation

Hemorrhagic Cysts

• Internal reticular/lace-like pattern (fibrin strands)
• No internal Doppler flow (distinguishes from solid lesion)
• Acute hemorrhage: hyperechoic; resolving: complex with settling debris
• Most resolve within 6–8 weeks

Dermoid (Mature Cystic Teratoma)

• Heterogeneous echotexture with echogenic components
• Dermoid plug (Rokitansky nodule): echogenic mural nodule with acoustic shadowing
• Tip of the iceberg sign: echogenic anterior component shadows the posterior contents
• Doppler: minimal or no internal flow

Endometrioma

• Homogeneous low-level internal echoes ("ground glass")
• Thick wall, may be multilocular
• "Kissing ovaries" when bilateral endometriomas adhere in the cul-de-sac

IOTA Simple Rules (Complex Masses)

For characterizing adnexal masses, the IOTA simple rules provide a structured approach. Malignancy features (M-features) include irregular solid components, ascites, and bilateral lesions. Benign features (B-features) include unilocular cysts, solid components <7 mm, and acoustic shadowing. Document which features are present to support the interpreting physician's assessment.

Reporting Tips

A complete ovarian evaluation should document:

1. Size — all three dimensions and volume for each ovary
2. Follicle count — if AFC is requested, report total count and per ovary
3. Dominant follicle — size and side, if present
4. Corpus luteum — location, Doppler appearance
5. Any cysts or masses — size, morphology (simple vs. complex), Doppler flow
6. Free fluid — location and amount (physiologic vs. abnormal)

When documenting PCOM, report:

• Follicle number per ovary (FNPO) and total
• Ovarian volume bilaterally
• Whether criteria for polycystic ovarian morphology are met

Avoid the term "polycystic ovaries" without specifying the measurement basis — it has loose colloquial meaning and the interpreting physician needs the numbers to make a clinical determination.

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Reference: 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome; IOTA group publications; AIUM Practice Parameter for Ultrasound Examination of the Female Pelvis. Clinical decisions should involve the interpreting physician.