Folliculometry: which day of the cycle and how many visits
An app puts ovulation on day fourteen because that is where it falls in a 28-day cycle. Follicle tracking shows when it falls for you — and that can be several days out, which is the whole chance for that cycle.
What the scan is for
Folliculometry is a series of short transvaginal scans within one cycle. The doctor measures the growing follicle and the thickness of the lining, and sees three things at once: whether the follicle is maturing, when it will mature, and whether ovulation actually happened.
Ovulation sticks from the pharmacy detect the LH surge, after which ovulation usually follows within 36 hours. Usually — but not always: LH can rise without the follicle rupturing. A test cannot tell the difference; a scan shows it directly.
Which day to start
With a 28-day cycle the first scan falls on day 9–10, counting from the first day of your period.
For any other length there is a simple rule: cycle length minus 18. The phase after ovulation lasts about 14 days in almost every woman; it is the first half that varies — so you count back from the end, not forward from the start.
| Cycle length | First scan | Ovulation expected |
|---|---|---|
| 26 days | day 8 | around day 12 |
| 28 days | day 10 | around day 14 |
| 32 days | day 14 | around day 18 |
| 35 days | day 17 | around day 21 |
How many visits, and what is seen
Usually two to four scans, every second day. A follicle grows by one and a half to two millimetres a day, so scanning daily adds nothing and scanning every third day can miss ovulation.
| What the doctor sees | What it means |
|---|---|
| Follicle 10–12 mm | a dominant follicle has emerged — from here it grows alone and the rest regress |
| Follicle 18–24 mm | mature; ovulation expected within a day or two |
| Endometrium 8 mm or more | the lining is ready for implantation |
| Follicle gone, free fluid appears | ovulation has happened |
| Corpus luteum | confirmation of ovulation in the second half of the cycle |
How you know ovulation happened
A large follicle proves nothing on its own — its disappearance does. On the scan afterwards the follicle is gone or collapsed, there is a small amount of free fluid in the pouch of Douglas, and a corpus luteum appears in the ovary.
A progesterone level 7 days after expected ovulation is checked as well — day 21 in a 28-day cycle. A raised value confirms that ovulation occurred and the corpus luteum is working.
With an irregular cycle
The "minus 18" rule does not work then, because there is nothing to subtract from. Scanning starts earlier, around day 7–8, and is repeated more often. Some cycles have no ovulation at all — this happens to healthy women once or twice a year, and much more often with polycystic ovary syndrome.
One cycle without ovulation is not a diagnosis. Two or three in a row is a reason to look for the cause: thyroid, prolactin, androgens and weight are checked.
When tracking alone is not enough
The scan answers "when", not "why it is not working". If you have been trying for a year — six months after 35 — tubal patency testing and a semen analysis for your partner are added. Half the causes of infertility are male, and a semen analysis is cheaper and faster than most of what a woman goes through.
Short answers
Which day of the cycle does folliculometry start?
Day 9–10 with a 28-day cycle. For other lengths: cycle length minus 18. With an irregular cycle, from day 7–8 with more frequent checks.
How many visits does it take?
Usually two to four in one cycle, every second day. The follicle grows 1.5–2 mm a day, so that interval is enough without missing ovulation.
At what follicle size does ovulation happen?
Most often at 18–24 mm. The size alone does not prove ovulation — that is proved by the follicle disappearing, free fluid, and a corpus luteum on the next scan.
Does folliculometry hurt?
No. It is an ordinary transvaginal scan, shorter than a full examination — a few minutes, with no preparation beyond an empty bladder.
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.