Dominant follicle: how many millimeters, and when you will ovulate
The scan report says “dominant follicle 14 mm”, and the question follows at once: is that a lot or a little, and when should ovulation come? Here is what this follicle is, how fast it grows, how many days it usually takes to rupture at each size, and when size promises nothing at all.
What a dominant follicle is
A follicle is a small fluid-filled sac in the ovary in which an egg matures. At the start of every cycle several small antral follicles of 2 to 9 mm grow at once. Around day 7 to 9 one of them pulls ahead and keeps growing while the rest stop and regress. That leader is the dominant follicle.
On ultrasound it is recognized by size: usually 10–14 mm when it emerges, and clearly larger than its neighbours. If everything goes the usual way, it is the one that will ovulate this cycle.
Which ovary it grows in, right or left, does not matter. The ovaries do not take turns on a schedule, and two cycles in a row on the same side is normal too.
How fast it grows
Once a follicle has become dominant it grows by roughly 1.5–2 mm a day. Ovulation happens when it reaches 18–25 mm, most often around 20–22. This “own” size is fairly constant for each woman from one cycle to the next.
The endometrium is checked together with the follicle. Before ovulation it becomes three-layered and about 8–12 mm thick. The whole picture of the cycle — follicle, fluid, corpus luteum — is in the article on ovulation on ultrasound.
Follicle N mm: when will I ovulate
The table below answers the most common question and is worked out from the growth rate. It shows how many days are usually left before ovulation at each size of the dominant follicle.
| Follicle size | Ovulation usually |
|---|---|
| 9–10 mm | in 5–7 days; the dominant one may not have emerged yet |
| 11–12 mm | in 4–6 days |
| 13–14 mm | in 3–5 days |
| 15–16 mm | in 2–4 days |
| 17–18 mm | in 1–3 days |
| 19–21 mm | within the next 1–2 days |
| 22–25 mm | mature; ovulation can happen at any moment |
| over 25 mm | checked for turning into a cyst: repeat scan in 1–2 days |
| over 30 mm | no longer a follicle but a follicular cyst |
If you are also using LH ovulation tests: after a positive test, ovulation usually follows within 36 hours. Together with ultrasound this is the most precise way to catch the right day without extra visits.
How to tell it has ruptured
A large follicle does not prove ovulation on its own — its disappearance does. A day or two after ovulation the scan shows:
- the follicle has gone, or collapsed and lost its smooth outline;
- a small amount of free fluid in the pouch of Douglas;
- a corpus luteum in its place — a structure with an irregular wall and a ring of blood flow around the edge.
Telling a dominant follicle from a corpus luteum on screen is not hard: the follicle has a thin, smooth wall and uniform dark fluid inside, while the corpus luteum has a thick, “crumpled” wall and may contain strands and blood.
No dominant follicle, two of them, or one that does not rupture
- No dominant follicle. All follicles stay small and the cycle passes without ovulation. One or two such cycles a year happen to any healthy woman. If it happens several cycles in a row, the cause is looked for: thyroid, prolactin, polycystic ovary syndrome, sharp changes in weight.
- Two dominant follicles. This happens even without stimulation. If both rupture, twins are possible — each baby then has its own placenta.
- The follicle grew but did not rupture. It keeps enlarging or turns into a corpus luteum without releasing the egg. Ovulation tests are positive and progesterone rises — only ultrasound can tell the difference.
- The follicle passed 30 mm. That is a follicular cyst; it usually resolves on its own within one to three cycles. Which cysts do not — in the article on ovarian ultrasound.
None of these findings is a diagnosis from a single cycle. They are assessed over two or three cycles in a row.
Antral follicles: how many there should be
The small follicles at the start of the cycle are counted separately — this is the antral follicle count (AFC), one of the measures of ovarian reserve. In women aged 25–35 both ovaries together usually show around 10–20. Clearly fewer is a reason to check AMH, especially if pregnancy is not planned right away. 20 or more in one ovary is one of the features of polycystic ovaries, but only together with the cycle and hormones, not on its own.
They are counted properly on day 2–5 of the cycle. What else to check before pregnancy — in the article on tests before pregnancy.
When to come for a scan
If the aim is to catch ovulation, the first scan in a 28-day cycle is on day 9–10; in a cycle of another length, on day “cycle length minus 18”. Then every one to two days until the follicle ruptures, and one visit afterwards to confirm it. How to count the days for different cycles — in the article on folliculometry.
I do follicle tracking myself, transvaginally, at my practice in Warsaw at ul. Głębocka 9, held in Polish, Russian and Ukrainian. No referral or NFZ insurance needed.
Short answers
What is a dominant follicle?
It is the follicle that pulls ahead of the others around day 7–9 of the cycle and keeps growing until it ovulates. On ultrasound it usually becomes distinguishable at 10–14 mm.
What size does a follicle ovulate at?
Most often at 18–25 mm, usually around 20–22 mm. A follicle grows 1.5–2 mm a day, so its size lets you estimate how many days are left.
Follicle 20 mm — when will I ovulate?
Usually within the next one or two days. A repeat scan the next day or a positive LH test is more precise: ovulation follows within 36 hours of it.
Follicle 15 mm — when will I ovulate?
Usually in 2–4 days: 5–7 mm remain to a mature 20–22 mm at a growth rate of 1.5–2 mm a day.
How do you tell a dominant follicle from a corpus luteum on ultrasound?
The follicle has a thin smooth wall and uniform dark fluid inside. The corpus luteum has a thick crumpled wall, may contain strands or blood, and shows a ring of blood flow around the edge on color Doppler.
Can a follicle grow and not rupture?
Yes. It keeps growing or turns into a corpus luteum without releasing the egg, even though ovulation tests are positive. Only an ultrasound a day or two after the expected ovulation can show this.
Sources
The primary sources this text rests on. Links open in a new tab.
- Trying to get pregnantNHS, UK — when ovulation happens and how to count the second half of the cycle
- Polyendocrine metabolic ovarian syndrome (PMOS)NHS, UK — polycystic ovary syndrome (now renamed PMOS), the most common cause of cycles without ovulation
- InfertilityWorld Health Organization — the definition of infertility and where the work-up starts
- Rekomendacje Polskiego Towarzystwa Ginekologów i PołożnikówPTGiP — the standard of care in Poland; my ultrasound skills certificate is issued under the same rules
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.
