Polycystic ovaries

Last revised by Dr Jeremy Jones on 20 Sep 2021

Polycystic ovaries (PCO) or polycystic ovarian morphology is an imaging descriptor of a particular type of change in ovarian morphology. A proportion of women with polycystic ovaries will have the polycystic ovarian syndrome (PCOS), which in turn requires additional clinical, as well as biochemical, criteria; otherwise polycystic ovaries can be considered a normal variant

Polycystic ovaries may be seen in ~20% women of reproductive age 1,4.

Transvaginal ultrasound is considered the gold standard in the diagnosis of polycystic ovaries. Features can affect either one (unilateral polycystic ovary) or both ovaries.

Features include:

  • increased follicle number per ovary (FNPO) - usually 20 or greater
  • individual follicles are generally similar in size and measure 2-9 mm in diameter 7
  • peripheral distribution of follicles; this can give a "string of pearls" appearance
  • background ovarian enlargement (volume greater than 10 mL) 7
  • central stromal brightness +/- prominence

Only one ovary fitting these criteria is sufficient to define PCO. If there is evidence of a dominant follicle (>10 mm) or a corpus luteum, the scan should be repeated during the next cycle 9.

Recent international PCOS consensus statement defines FNPO as 20 or more follicles per ovary 15, though earlier publications had suggested a more conservative cut-off of 25 follicles 14.  It is now generally accepted that the initial Rotterdam recommendation of 12 follicles per ovary warrants revision, based on limited initial evidence and advances in ultrasound technology allowing better visualization of small follicles.

A study published in 2013 suggested that the number of follicles in each ovary, the follicle number per ovary (FNPO) is a more sensitive and specific criteria for diagnosis of PCO, as compared with the numbers of follicles in cross section, also known as follicle number per section (FNPS), or ovarian volume. This study showed an FNPO threshold of 26 follicles had a sensitivity of 85% and specificity of 94% when discriminating between controls and PCOS. A FNPS threshold of nine follicles had a 69% sensitivity and 90% specificity, and an ovarian volume of 10 mL had a 81% sensitivity and 84% specificity 11.

Pelvic MRI may show most or all of the above sonographic features. Signal characteristics include:

  • T1: small uniform follicles are low in signal while the central stroma is of intermediate signal (vs normal myometrium)
  • T2: follicles have high T2 signal while the central stroma is of comparatively low T2 signal 8

General imaging differential considerations include:

  • multifollicular ovaries (MFO): fewer cysts (~6 or more per ovary), which tend to be larger (up to 10 mm in diameter) and distributed throughout the ovary with no stromal hypertrophy 2
  • with a lack of consensus sometimes it is easier to report the number of follicles in each ovary rather than attempt to label the ovaries as "polycystic" or "multifollicular"

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Cases and figures

  • Case 1: with polycystic ovarian syndrome
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  • Case 2
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  • Case 3
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  • Case 4
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  • Case 5
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