PCO stands for polycystic ovaries, a term describing the appearance of ovaries on an ultrasound scan. It means one or both ovaries contain a higher-than-usual number of small fluid-filled sacs (follicles), giving the ovary a characteristic “polycystic” look. PCO is extremely common and, on its own, is not a disease. It becomes clinically significant when it appears alongside other hormonal symptoms, at which point the diagnosis shifts to PCOS, or polycystic ovary syndrome.
What PCO Looks Like on Ultrasound
When a doctor describes your ovaries as “polycystic,” they’re referring to a specific pattern visible during an ultrasound. Under current international guidelines, PCO is defined as having 20 or more follicles in at least one ovary, or an ovarian volume greater than 10 milliliters. The ovaries may also appear slightly larger than average. Despite the name, these aren’t true cysts. They’re tiny follicles, each holding an immature egg, clustered around the edges of the ovary.
Older ultrasound equipment sometimes can’t count individual follicles precisely. In those cases, clinicians use the ovarian volume measurement or count the follicles visible in a single cross-section of the ovary (10 or more per section qualifies). The scan is typically done through the vagina, which gives a much clearer image than an abdominal ultrasound.
How Common PCO Is
Having polycystic ovaries is far more common than most people realize. Roughly 20 to 25 percent of women of childbearing age have the pattern on ultrasound. Some research puts the number even higher: polycystic-looking ovaries have been found in as many as 62 percent of women who ovulate normally and have no symptoms at all. The appearance also tends to become less common with age, as follicle counts naturally decline over time.
In teenagers, a multifollicular ovary pattern can be completely normal and doesn’t necessarily indicate anything unusual. This is one reason clinicians are cautious about using ultrasound alone to diagnose reproductive conditions in younger patients.
PCO vs. PCOS: The Key Difference
This is the distinction that trips most people up. PCO describes an ovary’s appearance. PCOS is a hormonal condition that affects the whole body. You can have polycystic ovaries without having the syndrome, and you can even be diagnosed with PCOS without polycystic ovaries showing up on a scan.
PCOS is diagnosed when at least two of the following three features are present:
- Polycystic ovaries on ultrasound
- Irregular or absent periods, typically cycles longer than 35 days or missed periods
- Signs of excess androgens, meaning elevated levels of hormones like testosterone, which can show up as increased facial or body hair, persistent acne, or thinning hair on the scalp
The critical difference is that PCOS involves the overproduction of androgens (often called “male hormones,” though all women produce them in small amounts). This hormonal imbalance is what drives most of the symptoms. It also disrupts ovulation: the ovaries try to release eggs, but the excess testosterone stalls the process, leaving follicles stuck at an immature stage. That’s what creates the polycystic appearance in the first place.
PCOS affects roughly 10 percent of women in Western countries, compared to the much larger percentage who simply have polycystic-looking ovaries. Before diagnosing PCOS, doctors need to rule out other hormone disorders that can mimic the same symptoms, such as thyroid problems or elevated prolactin levels.
Does PCO Alone Cause Symptoms?
If your ultrasound shows polycystic ovaries but your periods are regular and your hormone levels are normal, you likely won’t experience any symptoms. PCO on its own doesn’t cause acne, excess hair growth, or weight gain. Those symptoms are driven by the androgen excess and insulin-related changes that characterize PCOS.
The ultrasound finding by itself also doesn’t mean your ovaries aren’t working properly. Many women with polycystic ovaries ovulate on a regular cycle and conceive without difficulty. The follicles visible on the scan simply reflect a larger-than-average egg reserve, which isn’t inherently a problem.
PCO, Fertility, and What to Expect
If you have PCO without the full syndrome, your fertility outlook is generally the same as anyone else’s. The follicles on your ovaries represent eggs in reserve, and having more of them isn’t a disadvantage.
When PCOS is present, the picture changes. Irregular ovulation makes timing conception harder. In the general population, about 85 percent of couples conceive within a year of regular unprotected sex. For women with PCOS, that number drops to around 50 percent over the same timeframe. However, women with PCOS tend to maintain their egg reserve longer than average, which means their fertile years often extend further into their late 30s and early 40s. The path to pregnancy may take longer, but the overall capacity for it can persist later in life.
When PCO Matters More
A PCO finding on ultrasound is worth paying attention to if it appears alongside other changes. Cycles that have become noticeably longer or more unpredictable, new or worsening acne, or hair growing in places it didn’t before are all signs that the hormonal side of the equation may be shifting. In those cases, blood tests measuring hormone levels can clarify whether you’ve crossed into PCOS territory.
PCOS is closely linked to how the body handles insulin, the hormone that regulates blood sugar. Many women with the syndrome develop insulin resistance, which can increase the risk of type 2 diabetes and other metabolic issues over time. This metabolic connection is why PCOS is treated as a broader health condition, not just a reproductive one. Isolated PCO without hormonal or metabolic changes doesn’t carry these same risks.
If you’ve been told you have polycystic ovaries, the most useful next step is understanding which category you fall into. A simple blood panel checking androgen levels, along with a review of your menstrual cycle patterns, is usually enough to determine whether the finding is incidental or part of something that needs ongoing management.