When you are trying for a baby, waiting to know whether you are ovulating can feel like the longest part of each month. A follicular study takes away some of that guesswork. It lets you and your doctor watch, scan by scan, how an egg is growing in your ovary.
This guide explains what the scan is, which days it is done, what follicle size is considered mature, how to read the terms on your report and what may come next. It is written for anyone who has been advised this test.
Quick answer: A follicular study (also called follicular monitoring or folliculometry) is a series of transvaginal ultrasound scans that track how follicles grow in the ovaries during one cycle. Scans usually start around day 9-10 of a natural or IUI cycle and are repeated every few days. A follicle is usually considered mature at about 18-22 mm, and later scans can show whether it has released the egg.
Every month, a few small fluid-filled sacs called follicles start to grow in the ovaries. Each one holds an immature egg. Usually one follicle grows ahead of the others and becomes the dominant follicle. When it is mature, it releases the egg. This is ovulation.
A follicular study follows this process with ultrasound. According to ISUOG (the International Society of Ultrasound in Obstetrics and Gynecology), it can be used to:
UK national guidance (NICE NG257) recommends ultrasound monitoring of follicle size and number for women taking gonadotrophin injections, to lower the risk of a multiple pregnancy and of ovarian hyperstimulation syndrome (OHSS). So the scan is not only about timing, it is also a safety check.
At each visit the sonologist or doctor looks at three main things:
If you want to understand the lining measurement on your report, our guide on endometrial thickness explains what the numbers mean.
Day 1 is the first day of full bleeding of your period. The exact plan depends on your cycle length and treatment, but ISUOG describes this usual pattern:
| Type of cycle | Usual first scan | What happens next |
|---|---|---|
| Natural cycle (no medicines) | Around day 9-10 | Repeat scans every few days until the follicle is mature, then a scan to check it has ruptured |
| Ovulation medicines (tablets or injections) | Around day 9-10, sometimes a baseline scan on day 2-5 | Dose may be adjusted; trigger injection timed by follicle size |
| IUI cycle | Around day 9-10 | IUI is timed after the trigger or natural LH surge |
| IVF stimulation | Around day 5-6 of injections | Scans every 1-3 days until egg collection is planned |
If your cycles are long or irregular, the first scan may be later. In a 21-35 day cycle, ASRM notes that regular periods within this range are a good sign that ovulation is happening, but only a scan or a blood test can confirm it.
Each scan takes about 10-15 minutes, according to ISUOG. Most couples need three to five visits in one cycle. It helps to keep a simple diary of your cycle dates and scan results.
Once a dominant follicle appears, ISUOG says it usually grows by about 1-2 mm a day. It is generally considered mature at about 18-22 mm. ASRM gives a similar range, noting that ovulation generally happens when the follicle is about 1.8-2.5 cm (18-25 mm).
People often search for a "day 10" or "day 14" size. A rough guide in a regular 28-day cycle is below, but remember that every woman and every cycle is different:
| Cycle day (28-day cycle) | Typical finding | What it means |
|---|---|---|
| Day 2-5 (baseline, if done) | Several small follicles, thin lining | Starting point; checks for leftover cysts |
| Day 9-10 | A dominant follicle often seen, about 10-14 mm | Growth has started; next scan planned |
| Day 12-13 | Dominant follicle about 15-18 mm | Approaching maturity |
| Day 14 (approximately) | About 18-22 mm | Mature; ovulation expected soon |
| After ovulation | Follicle collapsed, fluid behind the uterus | Suggests the egg has been released |
In a longer cycle, maturity may come on day 16-17 or later; ISUOG notes that it can vary with cycle length. A follicle that is "small for the day" on one scan is not a cause for alarm. The doctor looks at the trend across scans, not a single number.
Key points so far
Reports can look like a table of numbers and abbreviations. Here are terms you may see:
Please do not judge your report on your own. Ask your doctor to explain it, because the meaning depends on your cycle day, medicines and treatment plan.
Sometimes the scans show no dominant follicle, slow growth, or a follicle that grows but does not release the egg. This can happen now and then even in healthy cycles. If it happens repeatedly, it may point to an ovulation problem, for example with PCOS or thyroid problems. Our article on ovulation and its importance explains common causes.
Depending on your tests, your doctor may suggest ovulation medicines, a trigger injection, changes to the dose in the next cycle, or other fertility treatments. Every option should be explained clearly, with time for you to ask questions.
A transvaginal scan may feel a little uncomfortable but is usually not painful. You can ask for a female staff member to be present and for the scan to be stopped at any time.
A follicular study is one way to track ovulation. This table compares it with other common methods:
| Method | What it shows | Limitations |
|---|---|---|
| Follicular study (ultrasound) | Follicle growth, lining and signs of rupture | Needs several clinic visits in one cycle |
| Ovulation (LH) urine kits | The LH surge; ovulation usually follows within about 12-24 hours (ASRM) | Does not confirm the egg was released |
| Mid-luteal progesterone blood test | Whether ovulation probably happened | Only after the event |
| Cycle tracking (calendar, symptoms) | A rough fertile window | Less accurate with irregular cycles |
Follicular monitoring is one of the fertility services at Padmasri Hospitals & IVF Centre, described on our follicular monitoring page. It is often part of ovulation induction, IUI or IVF, listed under fertility services. If you are still deciding between IUI and IVF, this IUI vs IVF comparison may help.
For a consultation, call +91 80083 36677 or use our contact page to enquire about an appointment. Padmasri Hospitals & IVF Centre, 49-48-16/5, NH16, NGGOS Colony, Akkayyapalem, Visakhapatnam, Andhra Pradesh 530016.
In a natural or IUI cycle it usually starts around day 9-10 of the cycle, counting the first day of your period as day 1. In IVF it starts around day 5-6 of injections. Your doctor may also do a baseline scan on day 2-5.
A follicle is generally considered mature at about 18-22 mm. ASRM notes that ovulation generally happens at about 18-25 mm. Your doctor looks at the trend across scans, not one number.
Most women need about three to five scans in one cycle, but it depends on cycle length, medicines and how the follicle grows.
It is a transvaginal ultrasound that may feel slightly uncomfortable but is usually not painful. Each scan takes about 10-15 minutes, and you can ask for it to be paused at any time.
It means the dominant follicle seen earlier has collapsed or disappeared, often with a little fluid behind the uterus. This suggests that ovulation has taken place.
Yes, the scan does not stop you from conceiving. It is often used to time intercourse or IUI in that same cycle.
Written by Padmasri Hospitals & IVF Centre Editorial Team · Updated 11 November 2026
This article is for general information and does not replace a consultation with a qualified doctor.