Ovarian Stimulation

Ovarian stimulation is the use of medication to make the ovaries develop follicles that would not otherwise mature, and it sits at the start of almost every fertility treatment, from a single tablet taken at home to the injection schedule that precedes egg collection.

The medicines are effective and the risks are real and quantifiable. This page sets both out plainly, because the decision to start is one you should be able to make with the numbers in front of you.

On this page

• What ovarian stimulation is
• Two different goals
• The medicines used
• IVF protocols
• Monitoring during the cycle
• Multiple pregnancy: the main risk
• Ovarian hyperstimulation syndrome
• Other risks and side effects
• Why women respond differently
• What it cannot do
• Questions patients ask

What ovarian stimulation is

Every month the ovaries begin developing a group of follicles, and normally only one becomes dominant while the rest are lost. Ovarian stimulation changes the hormonal signal so that either that one follicle matures reliably, or several mature together.

Which of those two things you want depends entirely on the treatment. Getting it the wrong way round is how avoidable harm happens, so the distinction is worth understanding before you start.

Two different goals

Ovulation induction. Used when you are not ovulating regularly — most often with polycystic ovary syndrome or other ovulation problems. The aim is one mature follicle, because fertilisation will happen in the body and the number of eggs released cannot be controlled afterwards.

Ovarian stimulation for intrauterine insemination follows the same logic, with one or at most two follicles as the target.

Controlled stimulation for IVF. Here the aim is deliberately multi-follicular: several eggs are collected so that more than one embryo can be created. Because the eggs are removed, the number of embryos put back is decided separately and can be limited to one.

The medicines used

Letrozole. An aromatase inhibitor taken as tablets early in the cycle. In polycystic ovary syndrome it produces ovulation more often than clomifene and is widely used first.

Clomifene citrate. Tablets that block oestrogen feedback at the brain so more follicle-stimulating hormone is released. It is inexpensive and familiar, and it can thin the endometrium and cervical mucus.

Gonadotropins. Injected FSH, or FSH with LH, acting directly on the ovary. Powerful, dose-sensitive, and the main route for IVF cycles. They require ultrasound monitoring; they are not medicines to take unsupervised.

Trigger injections. hCG or a GnRH agonist given to mature the eggs and time either ovulation or egg collection, usually 34 to 38 hours before the procedure.

Suppression drugs. GnRH antagonists or agonists that stop a premature natural surge from releasing the eggs before collection.

Ovarian stimulation protocols for IVF

The antagonist protocol is the common default: stimulation from early in the cycle, with an antagonist added once follicles reach a certain size. It is shorter and allows an agonist trigger, which sharply reduces hyperstimulation risk.

Long agonist protocols start suppression in the preceding cycle and still suit some women. Mild or low-dose protocols aim for fewer eggs with fewer injections and are used for particular indications rather than universally.

The ESHRE guideline on ovarian stimulation for IVF and ICSI reviews the evidence behind these choices. No protocol is right for everyone, and both partners are evaluated before any plan is made.

Monitoring during the cycle

Monitoring is what makes ovarian stimulation safe. Transvaginal ultrasound counts and measures follicles every few days, and blood oestradiol may be checked alongside.

Monitoring answers three questions: are enough follicles growing, are too many growing, and is the lining developing. Doses are adjusted on those answers, not on a fixed schedule.

Sometimes the right decision is to stop. A cycle may be cancelled for too few follicles or converted to a freeze-all, or the trigger withheld, when too many are developing. Cancellation is disappointing; it is also the mechanism that prevents the serious complications below.

Multiple pregnancy: the main risk

This is the risk most often underplayed, and it is the one that harms most often. Twin and higher-order pregnancies carry substantially increased risks of premature birth, low birth weight, admission to neonatal intensive care, pre-eclampsia, gestational diabetes, haemorrhage and caesarean delivery.

With clomifene or letrozole, published figures put the multiple pregnancy rate at roughly 5 to 10% of pregnancies. With injected gonadotropins used for ovulation induction or insemination, the reported figure rises to around 30%.

That is why a cycle is cancelled or converted when too many follicles develop. Proceeding with insemination when four or five follicles are mature is not an acceptable trade.

In IVF the control point is different and better: eggs are collected, and the number of embryos transferred is a decision. Transferring a single embryo is the single most effective way to avoid a twin pregnancy, and freezing the remainder means it costs you nothing in future attempts.

Ovarian hyperstimulation syndrome

OHSS is an exaggerated response in which the ovaries enlarge and fluid shifts out of the bloodstream into the abdomen. Published series report it in roughly 3 to 5% of stimulated cycles, with severe cases considerably less common.

Mild OHSS means bloating, abdominal discomfort and nausea. Severe OHSS means rapid weight gain, breathlessness, reduced urine output, marked abdominal swelling, vomiting and, rarely, blood clots. It needs hospital assessment, not reassurance over the phone.

Risk is higher in polycystic ovaries, a high antral follicle count, a high AMH, younger age, previous OHSS and a large number of follicles. The RCOG green-top guideline on OHSS sets out recognition and management in full.

Prevention is now effective: antagonist protocols, agonist triggers, dose reduction, freezing all embryos rather than transferring fresh, and cancelling when indicated. Report breathlessness, sudden weight gain or severe pain the same day.

Other risks and side effects

Common and usually tolerable: hot flushes, mood changes, headaches, breast tenderness, bloating, bruising at injection sites, and visual disturbance with clomifene, which is a reason to stop the drug.

Less common: ovarian torsion, in which an enlarged ovary twists and causes sudden severe pain requiring urgent surgery; pelvic infection or bleeding after egg collection; and allergic reaction.

Large studies have not established a causal link between fertility drugs and ovarian or breast cancer, though follow-up continues. The psychological load is routinely underestimated, and saying that you are struggling should change the plan, not just be noted.

Why women respond differently

Response is broadly predicted by age, antral follicle count and AMH. Ovarian reserve testing before starting allows the starting dose to be chosen rather than guessed.

Low responders produce few eggs despite high doses; simply raising the dose further rarely helps, and alternative approaches are discussed instead. High responders need the opposite care, because they are the group at risk of OHSS.

Body weight, thyroid function, smoking and previous ovarian surgery all influence the response, and some are modifiable before a cycle starts.

What it cannot do

Ovarian stimulation recruits follicles that were already present this month. It does not create new eggs, improve egg quality, or reverse the effect of age on the chromosomes inside them.

It also cannot compensate for blocked tubes or a significant sperm problem, which is why stimulation belongs inside a plan built on IVF or insemination rather than being tried repeatedly on its own. Add-ons are offered selectively here, not routinely.

Questions patients ask

Do the injections hurt? They are small subcutaneous injections into the abdomen or thigh. Most women describe them as a sting, and self-injection becomes routine within days.

How many cycles of tablets are reasonable? Ovulation induction is usually reviewed after about six ovulatory cycles. Continuing indefinitely without reassessment is not.

Will stimulation bring my menopause forward? No. It matures follicles that would have been lost that month anyway.

What should I ask before starting? How many follicles the plan is aiming for, at what point the cycle would be cancelled, which trigger is planned and why, and what number the team will ring if you feel unwell at the weekend. If those four answers are clear, the cycle is being run safely.

Written and medically reviewed by Dr. Vani Sundarapandian, MD, DGO, FRCOG (UK) — founder and Medical Director, Jananam Fertility Centre, Chennai.