In a natural cycle, several dozen follicles begin growing simultaneously each month. The body selects just one — the dominant follicle — and the rest atrophy. This isn't wasteful; one mature egg per month is entirely sufficient when everything works as it should.
IVF operates on different logic. The journey from egg retrieval to embryo transfer involves several stages, and at each one some eggs are lost: not all are mature, not all fertilise, not all embryos survive to blastocyst. The more eggs you start with, the better the odds that at least one good embryo reaches transfer.
Stimulation 'rescues' the follicles that would otherwise have atrophied. It doesn't create new eggs and doesn't borrow them from the future — it makes use of what was already in the ovaries but was destined to disappear. This matters a great deal to anyone worried about 'using up' their supply.
The foundation of stimulation is gonadotrophins — hormonal preparations containing FSH (follicle-stimulating hormone), the same hormone the pituitary gland produces in a natural cycle to drive follicle growth. In stimulation, FSH is delivered externally and in considerably larger amounts than the body would produce, persuading the ovaries to develop several follicles rather than one.
Alongside this, a GnRH agonist or antagonist is usually added. Its job is to prevent a premature LH surge — the signal that in a natural cycle triggers ovulation. Without this control, eggs could release on their own before retrieval.
Doses and specific medications are tailored to the individual. They depend on age, ovarian reserve (AMH), previous cycle outcomes and predicted ovarian response. A woman with PCOS and a woman with diminished reserve will be on fundamentally different protocols.
Roughly every two to three days during stimulation, an ultrasound monitoring scan is performed. The doctor counts the follicles and measures their diameter. The aim is to track how the ovaries are responding and adjust the medication dose if needed.
The optimal moment for retrieval is when enough follicles have reached 18 to 20 mm in diameter. At that size, the egg inside is most likely to be mature. Larger, and the egg may be over-mature; smaller, and it may not be ready. The clinician is threading a needle: timing matters.
Blood is also taken for oestradiol — its level reflects how active the growing follicles are, and helps assess the risk of hyperstimulation.
When the follicles are ready, a 'trigger' injection is given — a shot of medication that initiates the egg's final maturation. Without it, the egg doesn't complete meiosis and is technically not ready for fertilisation.
The standard trigger is hCG (human chorionic gonadotropin), which mimics the natural LH peak and sets off the same cascade of processes. The alternative is a GnRH agonist trigger, used when the risk of ovarian hyperstimulation syndrome (OHSS) is high — in patients with PCOS or with a very strong ovarian response.
This part is non-negotiable: retrieval must happen exactly 36 hours after the trigger. Not roughly, not approximately — exactly. Because around 38 to 40 hours post-trigger, spontaneous ovulation begins. The window for retrieval is narrow.
Stimulation is a significant hormonal intervention, and how people respond to it varies. Bloating and abdominal discomfort are almost universal. The ovaries grow considerably — sometimes to several times their normal size — and that is noticeable. A feeling of heaviness or fullness in the lower abdomen during the final days of stimulation is entirely expected.
Mood swings, irritability, tearfulness — these are the result of elevated oestrogen levels and the general physical burden of the protocol. They are not 'nerves'. They are physiology.
Headaches, fatigue, breast tenderness occur less often but are not unusual.
What is not normal and requires immediate contact with the clinic: severe or sudden abdominal pain, significant abdominal swelling, sudden breathlessness, or a marked reduction in urination. These are signs of possible severe OHSS. With modern protocols, severe forms are rare — but knowing what to watch for matters.
OHSS is a complication of stimulation in which the ovaries respond excessively. Blood vessels become more permeable, and fluid leaks into the abdominal cavity. Mild forms — bloating and discomfort — resolve on their own. Moderate cases require monitoring and sometimes hospitalisation. Severe forms are rare but serious.
Risk factors: PCOS, young age, high AMH, a large follicle count. When these are present, the clinician will choose a more cautious protocol and typically use a GnRH agonist rather than hCG as the trigger — this significantly reduces the risk of severe OHSS. If risk remains high, a freeze-all strategy is used: all embryos are frozen, the transfer is postponed to the next cycle once the ovaries have settled.
Almost everyone wants to know this. A typical 'good' response is 8 to 15 mature eggs. But 'good' is individual. For a 38-year-old with an AMH of 0.8, retrieving 5 eggs is an excellent outcome. For a 29-year-old with a high AMH, 20 eggs may be normal — though it also raises the risk of OHSS.
Egg count is not the same as embryo count, and embryo count is not the same as the number of children. Between 'eggs retrieved' and 'baby born' there are several stages of attrition. But more eggs means more attempts, which means a higher cumulative chance.
Stimulation is a technically demanding but well-established process. Clinics around the world perform it daily. The discomfort it causes is not a sign that something is going wrong — it is a predictable response to a physiologically intensive protocol.
The best things you can do during this phase: follow instructions exactly, especially regarding trigger timing; don't skip monitoring appointments; and tell your doctor about any symptoms that feel unusual or severe. Stimulation is teamwork.
AMH (Anti-Müllerian Hormone) — a marker of ovarian reserve. High AMH predicts a strong response but also raises the risk of OHSS.
Blastocyst — an embryo on days 5–6 of development. The optimal stage for transfer or freezing.
Dominant follicle — the follicle that 'wins' in a natural cycle. In a stimulated cycle, there are several.
Freeze-all — the strategy of freezing all embryos without a fresh transfer. Used when OHSS risk is high and in other clinical situations.
GnRH agonist — a medication that first stimulates, then suppresses FSH and LH production. Used both in long protocols (pituitary downregulation) and as an OHSS-sparing trigger.
GnRH antagonist — a medication that rapidly blocks GnRH receptors, preventing a premature LH surge. Used in short protocols, the most common approach today.
Gonadotrophins — hormonal FSH and/or LH preparations injected subcutaneously to stimulate follicle growth.
OHSS (Ovarian Hyperstimulation Syndrome) — an excessive ovarian response to hormone stimulation.
Trigger — an injection of hCG or GnRH agonist that initiates final egg maturation. Retrieval takes place exactly 36 hours later.
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