Ovulation Induction and Assisted Reproduction

The axis you are really prescribing to

Every fertility drug is a lever on one feedback loop — so learn the loop first. The hypothalamus releases GnRH in pulses; the pituitary answers with FSH and LH; the ovary answers those with a growing follicle and rising oestrogen. Oestrogen then feeds back to the brain, mostly turning the volume down — classic negative feedback — until a late surge of LH triggers ovulation. The Endocrine section teaches this circuitry as physiology; here we simply pull its levers. To make an anovulatory woman ovulate you need more FSH reaching her ovary. You can do that indirectly — by tricking the brain into sensing less oestrogen, so it releases more of its own FSH — or directly, by injecting FSH itself. Almost everything that follows is one of those two strategies.

Letrozole: starving the feedback signal

Letrozole is an aromatase inhibitor — the same class the Oncology section uses in breast cancer. Aromatase is the enzyme that converts androgens into oestrogen, so blocking it lowers circulating oestrogen for a few days. The brain, sensing that oestrogen has fallen, lifts its foot off the brake and releases more FSH, and that extra FSH coaxes a follicle to grow. Crucially, because letrozole is cleared quickly, oestrogen recovers in time for a healthy endometrium and cervical mucus. In modern practice letrozole has overtaken clomifene as first-line ovulation induction in PCOS: head-to-head trials show higher live-birth rates and fewer multiple pregnancies. It is the same drug the PCOS chapter reaches for, used here for the opposite goal — not to suppress a hormone-driven cancer but to gently release the ovary.

Clomifene: fooling the brain into panic

A selective oestrogen-receptor modulator that blindfolds the hypothalamus. Clomifene is a selective oestrogen-receptor modulator (SERM), a cousin of tamoxifen. Given for five days early in the cycle, it blocks oestrogen receptors in the hypothalamus. The brain can no longer feel the circulating oestrogen, so it wrongly concludes the ovary is quiet and ramps up GnRH — and therefore FSH and LH — to compensate. That surge of FSH drives a follicle to maturity and ovulation follows. It is an elegant trick: you produce more of the body's own gonadotropins without injecting anything. The catch is that clomifene blocks oestrogen receptors everywhere, not just the brain, so its anti-oestrogenic action can thin the endometrium and dry cervical mucus — occasionally undermining the very pregnancy it is trying to create. Response is monitored with ultrasound, and because more than one follicle may ripen, there is a real risk of twins.

Metformin and the resistant ovary

Because PCOS is driven in large part by insulin resistance, metformin — the same insulin-sensitiser the Endocrine section teaches for type 2 diabetes — is used as an adjunct. By lowering insulin it reduces ovarian androgen production and can help restore ovulation, sometimes added to letrozole or clomifene in women who do not respond alone (particularly those who are overweight). It is not a first-line ovulation inducer on its own, but it earns its place as a partner drug and a reminder that the metabolic and reproductive halves of PCOS are one problem, covered together in the PCOS chapter.

Gonadotropins: injecting the signal directly

When oral agents fail — the clomifene- or letrozole-resistant patient — or when many eggs are wanted for IVF, the strategy switches from persuading the pituitary to bypassing it. Injectable gonadotropins deliver FSH (with or without LH) straight to the ovary, driving follicles directly. This is far more powerful and far less forgiving: the dose–response curve is steep, so growth must be tracked closely with ultrasound and serum oestradiol, and the cycle cancelled or the dose cut if too many follicles appear. The same gonadotropin principle reappears in the Male-health chapter, where FSH and hCG are given to restart spermatogenesis in men with hypogonadotropic hypogonadism — the axis run in reverse to make sperm rather than eggs.

IVF: controlled ovarian stimulation, step by step

An IVF cycle is choreography of the same axis — grow many eggs, hold back the surge, then trigger. For in-vitro fertilisation the goal is not one egg but many, so high-dose gonadotropins grow a whole cohort of follicles at once. The danger during this stimulation is that the rising oestrogen could set off a premature LH surge and ovulate the eggs before they can be retrieved. To prevent that, the pituitary is deliberately suppressed. There are two ways to do it, and the choice is the recurring GnRH agonist-versus-antagonist theme you already met in the Endometriosis and Prostate chapters. A GnRH agonist first over-stimulates then down-regulates the pituitary (the older "long protocol") — effective but slow, since it takes a couple of weeks of initial flare to achieve suppression. A GnRH antagonist blocks the receptor immediately, shutting down LH within hours; this is the modern, shorter, more flexible protocol and the one that best allows a safer trigger.

Once enough follicles have matured, a single trigger completes the eggs' final maturation, mimicking the natural LH surge. Classically this is hCG, which acts on the LH receptor; but on an antagonist protocol you can instead trigger with a bolus of a GnRH agonist, which prompts the woman's own pituitary to release a short, natural LH surge. That agonist-trigger option is important for safety, as we are about to see. Eggs are retrieved, fertilised, and after transfer the corpus luteum is supported with progesterone (the stimulation protocols leave natural luteal support inadequate), often continued into early pregnancy — the same progesterone the Contraception and Menopause chapters describe, here used to hold rather than prevent a pregnancy.

The signature danger: ovarian hyperstimulation syndrome

Push the ovary too hard and it answers with a systemic, sometimes life-threatening leak. Ovarian hyperstimulation syndrome (OHSS) is the exaggerated response that gives this field its most feared complication. Many follicles, all luteinised by the hCG trigger, flood the circulation with vascular endothelial growth factor (VEGF). VEGF makes capillaries leaky, so fluid pours out of the vessels into the abdomen (ascites) and sometimes the chest. The ovaries swell dramatically, the intravascular volume drops, and the blood left behind becomes concentrated — haemoconcentration — which, combined with the thrombogenic pregnancy-like hormonal state, sharply raises the risk of venous thromboembolism (the clotting theme the Haematology section develops in full). In severe OHSS the picture becomes a genuine emergency: tense ascites, oliguria, electrolyte derangement, thrombosis, and rarely death. Note that hCG is the accelerant — both the trigger injection and, later, the hCG of an early pregnancy can ignite or worsen it, which is why OHSS can strike after a successful conception.

Prevention is pure pharmacology, and it is where the protocol choices above pay off. High responders — young women, those with PCOS or a high antral follicle count, and those whose oestradiol climbs steeply — are the classic at-risk group, so the first defence is titrating the gonadotropin dose to the individual. Using a GnRH-antagonist protocol lets you replace the hCG trigger with a GnRH-agonist trigger, which produces a short natural LH surge that matures the eggs without the prolonged luteal hCG stimulus that drives VEGF — dramatically cutting severe OHSS. A "freeze-all" strategy (freezing embryos and transferring in a later, unstimulated cycle) avoids the added hCG of an early pregnancy. And the dopamine agonist cabergoline can be given to blunt VEGF-mediated vascular permeability. The same VEGF pathway links back to the Oncology section, where anti-VEGF antibodies are used deliberately to starve tumours of blood supply — here the aim is the reverse, to calm a leak the drugs themselves provoked.