Follicle tracking scan: how ovulation is monitored with ultrasound

Each month the ovaries develop fluid-filled sacs called follicles, each containing an egg. Usually one becomes dominant and releases its egg, which is ovulation. Follicle tracking, also called follicular monitoring, is a series of ultrasound scans carried out at intervals across a cycle to watch the follicle or follicles grow and to see whether and when ovulation occurs.

The University Hospitals Coventry and Warwickshire fertility service explains that if a woman has an irregular menstrual cycle, monitoring with ultrasound scans, which it calls follicle tracking, and hormone assessments may help identify the fertile time of the month and so improve the chances of natural conception. The scans can also be used to see how the ovaries respond when fertility medicines are used.

It is used in several situations. The most common are irregular or absent periods, where timing intercourse is difficult; ovulation induction with tablets or injections, where the response must be watched; and some assisted conception treatments such as intrauterine insemination, where the timing of the procedure depends on follicle size.

The NHS says the most common causes of infertility include lack of regular ovulation, poor quality semen, blocked or damaged fallopian tubes and endometriosis. Lack of regular ovulation is the problem that follicle tracking addresses most directly. The UHCW page lists the most common causes of failure to ovulate as stress, weight fluctuations and polycystic ovary syndrome, with others including disorders of the pituitary or thyroid gland and raised prolactin.

Before ovulation problems are treated, the unit says tests are done to establish the cause, including an ultrasound scan of the ovaries and womb and blood tests for hormones such as thyroid hormones, prolactin, FSH, LH and testosterone.

The NHS describes two common ways of doing an ultrasound scan: external, with a probe moved across the skin, and internal, where a tube-shaped probe with a latex cover is gently placed into the vagina. Follicle tracking is often carried out with an internal scan. You may be asked to lie on a bed with your knees apart or on your side, and the probe is lubricated. The NHS says you can ask to insert the probe yourself, should tell staff if you have a latex allergy so an alternative can be used, and are in control and can ask the person to stop at any time.

The scan usually takes only a short time, and the healthcare professional will measure the follicles on the screen. Appointments are repeated across the cycle; how many depends on the reason for tracking, the medicines being used and the clinic’s protocol. Blood tests for hormones may be taken at the same visits. Follow your appointment letter for any preparation, as the NHS says some scans need a full bladder.

The UHCW unit explains that ovulation induction uses one of two main approaches: clomifene tablets, which increase the pituitary’s production of FSH and so stimulate follicles, or gonadotrophin injections, which contain FSH. The NHS lists clomifene, tamoxifen, metformin, gonadotrophins and other medicines among fertility medicines, some of which may cause side effects such as nausea, vomiting, headaches and hot flushes.

In the UHCW description, the response to the drugs is monitored by ultrasound scans; when follicles have reached an appropriate size, intercourse is advised or an injection of hCG is given to time intercourse or IUI. Individual responses can be unpredictable, and if the response is insufficient or too strong, the cycle may have to be cancelled and restarted. If the response is satisfactory, treatment usually continues for 6 cycles.

The unit notes that multiple pregnancy is a risk of ovulation induction: twins can result in up to 10% of cases with clomifene and 20% with gonadotrophins, with triplets in around 1%. Careful monitoring reduces the risk of multiple pregnancy but does not eliminate it. This is a principal reason for tracking.

Follicle tracking can show whether follicles are growing and how large they are, which is what UHCW uses to time intercourse, an hCG injection or IUI. It does not examine the other common causes of infertility the NHS lists, such as poor quality semen, blocked or damaged fallopian tubes and endometriosis, so tracking is one piece of a wider assessment.

It can also be time-consuming and can feel clinical, particularly when scans are frequent. Some people find it reassuring to know when ovulation happens; others find it adds pressure. Talk to your clinic about how many scans are really needed.

The NHS says fertility treatment offered depends on what is causing the problem and what your local integrated care board (ICB) funds. A GP can start the process by checking for common causes and arranging tests; see a GP if you have not conceived after a year of trying, or sooner if you are 36 or over. Treatment offered, including any monitoring, depends on the cause and on local funding.

Private fertility clinics and ultrasound providers offer tracking scans without a long wait, and some patients choose this to speed up an investigation. The NHS suggests asking about which treatments are offered, success rates, waiting lists and costs, and asking for a personalised, fully costed treatment plan that explains what is included, such as fees, scans and medicines, when comparing clinics.