Ovulation on ultrasound: the follicle, the corpus luteum and the fluid

Before ovulation: how the dominant follicle grows

Early in the cycle the ovaries show several small follicles of 2 to 9 mm. Around day seven to nine one of them pulls ahead — that is the dominant follicle. From then on it grows by roughly 2 mm a day, and that rate makes it possible to predict the day of ovulation several days in advance.

Ovulation happens when the follicle reaches 18 to 25 mm, most often around 20 to 22. That is the answer to “how many millimetres it should be”: not one number but a range, different for each woman and repeatable from cycle to cycle.

The endometrium, the lining of the uterus, is assessed at the same time. Before ovulation it takes on a three-layer appearance and a thickness of about 8 to 12 mm. A good follicle with a thin lining is a separate finding, and one worth discussing at the appointment.

In a cycle longer or shorter than 28 days the whole table shifts. It is easier to count backwards from the next period than forwards from the last one: the second half of the cycle is almost constant at about 14 days, and it is the first half that stretches or shortens.

Three signs that ovulation has happened

None of them proves ovulation on its own. Together they confirm it:

1. The follicle is gone, or has suddenly shrunk and lost its smooth outline. This is the most reliable sign — the disappearance of something that measured 22 mm yesterday.
2. Free fluid in the pouch of Douglas — a small amount released from the ruptured follicle. On its own it has other causes, but together with a vanished follicle it is confirmation.
3. A corpus luteum where the follicle used to be — a structure with an irregular interior and a characteristic ring of blood flow around its edge.

An easy mistake to make on your own: seeing “free fluid” in a report and assuming something is wrong. A small amount of fluid mid-cycle is normal, and is rather good news.

The corpus luteum: when it appears and what it looks like

The corpus luteum is what the ruptured follicle turns into. It is visible within a day or two of ovulation and lasts about two weeks, producing progesterone. If pregnancy does not occur it regresses and the period begins; if it does, the corpus luteum keeps working for several more weeks until the placenta takes over.

On screen it is usually a structure of 15 to 30 mm with uneven walls, sometimes with blood inside — which looks alarming and is still normal. Its hallmark is the ring of vessels around the rim, sometimes described as a “ring of fire”.

Hence a common misunderstanding: a report on day 20 of the cycle says “corpus luteum cyst, 25 mm”, and it reads like a diagnosis. In fact it is a normal stage of the cycle. It is checked simply — a repeat scan after the next period: the corpus luteum will have gone by then, a real cyst will still be there.

When the follicle grows but ovulation does not happen

It happens: the follicle reaches 20 mm or more and then does not rupture — it keeps enlarging, or gradually turns into a corpus luteum without releasing the egg. Such a cycle looks ovulatory on blood tests: progesterone rises and ovulation tests are positive. The only way to see the difference is to look at the follicle itself.

The second possibility is that the follicle overgrows and becomes a follicular cyst: the same structure, only larger than 30 mm. It usually resolves on its own within one to three cycles, and the repeat scan after a period is precisely a check that it has gone.

Neither scenario is a diagnosis after a single cycle. They are assessed over two or three consecutive cycles, because any healthy woman has one or two cycles a year without ovulation.

How monitoring works in practice

It is the same transvaginal ultrasound as at a routine gynecological appointment: no preparation, no full bladder, about ten minutes. The difference is that it is repeated several times within one cycle.

The usual pattern in a 28-day cycle: a first visit on day 9 to 11, then every one or two days until ovulation — two to four visits as a rule — and one afterwards to confirm it happened. With a long or irregular cycle there are more visits and they start later.

What to bring: the dates of your last periods for at least three cycles, and any previous results. A private appointment needs no referral and no NFZ insurance.

Short answers

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.