Ovarian ultrasound: cysts, reserve, and what “polycystic” means
The commonest finding in an ovary is a cyst, and the commonest cyst goes away by itself within one to three cycles. Telling that kind apart from the kind that needs watching can be done at the appointment — from its appearance, its size and the day of the cycle. Here is how it works and what to do with a report in hand.
How the ovaries are examined
The main approach is a transvaginal scan: the probe sits a couple of centimetres from the ovary, and the picture is as detailed as it can be. No preparation is needed, the bladder should be empty, and it takes about ten minutes.
The ovaries are examined through the abdomen when the transvaginal route is not appropriate: for women who have not had intercourse, and for large masses that do not fit in the field of view. In that case the bladder must be full instead.
Which day to book. For a clean picture, days 5 to 10, right after your period: at that point the ovary holds neither a dominant follicle nor a corpus luteum, so anything visible is either normal or a genuine finding. On day 20 that same corpus luteum is easily written up as a cyst.
A normal ovary in a woman of reproductive age is about 3 × 2 × 2 cm with a volume of up to 10 ml and several small follicles. After menopause it halves in size and no longer contains follicles.
Functional cysts — the ones that resolve by themselves
More than half of all ovarian cysts are not a disease but a cycle that overshot:
- Follicular cyst — a follicle that grew but did not rupture. Anything over 30 mm is already called a cyst. Thin wall, uniform contents, nothing inside.
- Corpus luteum cyst — a corpus luteum with blood inside. It looks more complicated: septations, a lacy interior and a ring of vessels around the rim. This is the one that frightens people most.
Both disappear on their own within one to three cycles. That is why the standard response to such a finding is not treatment but a repeat scan after the next period. Gone — the question is closed. Still there — it was not a functional cyst, and the search continues.
The same explains how one woman can be frightened twice and reassured once inside a month: the report depends on the day she happened to be scanned.
Cysts that do not resolve on their own
| What it is | How it looks on ultrasound | What follows |
|---|---|---|
| Endometrioma (“chocolate cyst”) | uniform ground-glass contents, no blood flow inside | observation or surgery — depending on size, pain and plans for pregnancy |
| Dermoid cyst | mixed contents: fat, hair, sometimes tooth buds; bright areas casting a shadow | usually removed, but without urgency: it will not resolve |
| Cystadenoma | thin walls, clear or mucinous contents, can grow large | observation or removal depending on size |
| Hydrosalpinx | an elongated space with incomplete septa — not the ovary but a dilated tube | matters when planning a pregnancy, discussed separately |
Certain features make a mass suspicious and prompt referral to a gynecological oncologist: thick irregular septations, solid growths inside, active blood flow within those growths, fluid in the abdomen, rapid growth. A thin-walled single-chamber cyst with no blood flow, meanwhile, is benign in the great majority of cases, whatever its size.
Polycystic ovaries and the syndrome are not the same thing
There is indeed an appearance on ultrasound called polycystic: twenty or more follicles of 2 to 9 mm in one ovary and an increased volume above 10 ml. By itself this means nothing yet.
One woman in five with a completely normal cycle has this appearance. Polycystic ovary syndrome is diagnosed only when at least two of three criteria are met: infrequent or absent ovulation, signs of excess androgens (on tests or clinically), and the ultrasound appearance. A scan alone is never enough.
Separately: in adolescents the diagnosis is not made from ultrasound at all — in the first years after menarche this appearance is normal.
Ovarian reserve
Reserve means how many eggs are left. It is assessed in two ways, and they complement each other:
- Antral follicle count on ultrasound — every follicle of 2 to 10 mm in both ovaries, counted on days 2 to 5 of the cycle;
- AMH (anti-Müllerian hormone) from a blood sample, which can be taken on any day.
Both describe quantity rather than quality, and neither answers the question “when will I run out of eggs”. Low reserve does not mean pregnancy is impossible, and high reserve does not guarantee it: they say only how much time is in hand and how the ovaries are likely to respond to stimulation.
Keep previous reports together. With a mass, what decides is often not size in itself but comparison: a 4 cm cyst stable for three years and a 4 cm cyst that was not there six months ago are two different situations.
Short answers
Which day of the cycle is best for an ovarian ultrasound?
Days 5 to 10, right after your period. At that point the ovary holds neither a dominant follicle nor a corpus luteum, so anything visible is either normal or a genuine finding. In the second half of the cycle a corpus luteum is easily mistaken for a cyst.
Does an ovarian cyst need surgery?
More than half of cysts are functional and resolve within one to three cycles — they are not treated but checked with a repeat scan after the next period. Surgery is discussed for cysts that do not resolve — endometrioma, dermoid, cystadenoma — and the decision rests on size, pain and plans for pregnancy.
What does “polycystic ovaries” on a scan mean?
Twenty or more follicles of 2 to 9 mm in one ovary, or a volume above 10 ml. It is an appearance, not a diagnosis: roughly one woman in five with a normal cycle has it. The syndrome is diagnosed only alongside disturbed ovulation or excess androgens.
How is ovarian reserve checked?
By two methods together: the antral follicle count on ultrasound on days 2 to 5, and an AMH blood test, which can be taken on any day. Both show the number of eggs but neither answers when menopause will arrive.
Can the ovaries be examined if I have not had intercourse?
Yes, through the abdomen. In that case the bladder must be full — drink about a litre of water an hour before the appointment and do not go to the toilet.
This text describes the general picture and does not replace a consultation. Timings and indications differ from woman to woman and are decided at the appointment, from your history and test results.
