Follicular study: why follicle scans are repeated in IUI and IVF, and what they measure
A follicular study is a short series of ultrasound scans that follows the follicles, the small fluid-filled sacs that hold the eggs, as they grow during a treatment cycle. The doctor checks how many are growing, how big they are and how the womb lining looks. Because follicles change every day, repeated scans tell us exactly when to act.
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In this short video, Dr Mayur Pai explains the follicular study in a simple way, as it is done while the ovaries are being stimulated. This guide explains why the scans are repeated, what is measured, and how that sets the timing of an IUI or an egg collection.
What is a follicle, and why watch it?
Each month, a group of follicles starts to grow in the ovaries. A follicle is a small sac of fluid, and inside each one is an egg. The egg itself is far too small to see on ultrasound, but the follicle around it is easy to see and measure. As the egg matures, its follicle grows, so the size of the follicle is our guide to how ready the egg is.
Normally one follicle takes the lead and releases its egg. When medicines are used to support the ovaries, more than one may grow, and in IVF we deliberately aim for several. Either way, the scans show what is really happening.
Why is one scan not enough?
A single scan is only a snapshot. Follicles grow a little every day, often around one to two millimetres a day once they are on their way, and every woman responds to treatment at her own pace. The right moment to act is a short window, so we need to see the trend, not just one picture. A typical follicular study looks like this:
- A baseline scan on the second or third day of the period, to check that the ovaries are quiet, that there is no leftover cyst, and to count the small resting follicles.
- Follow-up scans, usually starting after the first several days of treatment and repeated every one to three days, closer together as the follicles near their target size.
- A final check to decide on the trigger injection, and sometimes a later scan to confirm that the egg has been released.
What does the doctor measure on the screen?
- The number of follicles growing in each ovary.
- The size of each follicle, measured across in two directions and averaged, in millimetres. The largest is called the lead follicle.
- The womb lining, or endometrium: how thick it is, and whether it shows the layered, "triple-line" look of a lining that is responding well to the rising hormones.
- Signs that ovulation has happened on a later scan, such as a collapsed follicle and a little free fluid behind the womb.
- The overall size of the ovaries in stimulated cycles, as an early warning of over-response.
In IVF cycles, a blood test for hormones is sometimes added, so that the doctor has the full picture before deciding on the next step.
How do the scans set the timing?
When the lead follicle reaches the size at which its egg is usually mature, commonly somewhere around 18 to 20 millimetres, and the lining looks ready, the doctor plans the trigger injection. The exact size differs from patient to patient and between IUI and IVF, and the doctor decides by looking at the whole picture, not one number.
The trigger completes the final ripening of the egg and sets its release in motion, usually about a day and a half later. That predictable gap is what allows precise planning. In an IUI cycle, the insemination is timed to it; you can read what happens on IUI day. In a cycle with tablets alone, the scans tell you which days to try naturally. In IVF, egg collection is timed a little before the eggs would be released on their own.
Without scans, a trigger given too early would release an immature egg, and one given too late could miss the egg altogether. That is why a few short visits are worth the effort.
Scans in an IVF cycle
Watch on YouTube if the player does not load.
In the second video, Dr Mayur Pai explains the basics of follicle scanning during an IVF cycle. In IVF the aim is different from IUI: we want several follicles to grow together, so that several eggs can be collected. Scans are therefore more frequent, and each one guides a decision.
The scan tells the doctor whether the daily injections should continue unchanged or be adjusted, when to add the medicine that stops the eggs from being released too early, and when the follicles are ready for the trigger. It also shows whether the ovaries are responding too strongly. Over-response is a real risk, particularly with polycystic ovaries, where many small follicles can grow at once; our guide on egg development in PCOS explains why. Careful scanning, adjusting the medicines in time and, where needed, freezing the embryos for a later transfer are how the team keeps this risk low.
Scans in an IUI cycle
In IUI the aim is the opposite: one or two good follicles, not many. If the scan shows that too many follicles are growing, the doctor may advise cancelling that cycle or changing the plan, to avoid the risk of a multiple pregnancy such as triplets. This can feel disappointing, but it is a safety decision, and a well-watched cycle is a safer cycle for mother and baby. The wider treatment is explained in what IUI is and how it works.
What is the scan like for you?
A follicle scan is an internal, or transvaginal, ultrasound. A slim probe with a clean cover and gel is placed gently in the vagina, which gives a much clearer picture of the ovaries than a scan over the tummy. It takes a few minutes. Most women feel only mild pressure, and the bladder should usually be empty.
Ultrasound uses sound waves, not radiation, so repeating it during a cycle is safe for you and for the eggs. The baseline scan is often done while you are still bleeding, which is normal and does not spoil the picture. Scan visits are short, and the team tries to plan them around your work.
Where treatment happens
This guide is by Dr Mayur Pai, MBBS, MS (Obstetrics & Gynaecology), a fertility and laparoscopy specialist. Pai Hospital in Goa has its own operating theatre and its own IVF laboratory, and the doctors who read your scans also plan your treatment, so each decision is made with the full picture. For the wider steps of IVF, read how IVF works, step by step.
Questions, or ready to book?
If you have a question, send a WhatsApp message to +91 95185 02773 with your situation and any reports you already have. Consultations are by appointment, so please call Sheetal at our fertility reception on +91 95459 20817 to book a time. Consultations are held in Panjim, Margao, Mapusa and Vasco. This article is for education only and is not personal medical advice; please consult your doctor about your own situation.
Your questions
How many follicle scans are needed in one cycle?
It varies. An IUI cycle usually needs a handful of short scans, and an IVF cycle a few more, because the medicines are adjusted as you go. The team gives you the next date at each visit, depending on how the follicles are growing.
Is a follicle scan painful?
No anaesthesia is needed. It is an internal scan with a slim, covered probe, and most women feel only mild pressure for a few minutes.
Can the scan be done during my period?
Yes. The first, baseline scan is often done on the second or third day of the period, while you are still bleeding. Bleeding does not affect the picture.
Is it safe to have so many ultrasound scans?
Yes. Ultrasound uses sound waves, not radiation, so it is safe to repeat during a treatment cycle, for you and for the eggs.
What size should the follicle be before the trigger injection?
Commonly around 18 to 20 millimetres, but there is no single number for everyone. The doctor also looks at how many follicles there are, the womb lining and, in IVF, sometimes a blood test, before choosing the day.
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This page is for general education and is not personal medical advice. Treatment depends on your own examination and reports, so please speak to a doctor about your situation. We do not carry out or offer sex determination of any kind.