Every woman is born with all the eggs she will ever have. They form before birth, and no new eggs are made afterward. At birth, the ovaries hold roughly 1–2 million eggs. By puberty that's down to 300,000–400,000. Only 400–500 will ever be ovulated across a lifetime. This natural, ongoing decline in number (and, with it, quality) is called ovarian ageing. "Ovarian reserve" is the term for how many eggs remain at any given point. It's assessed with an AMH blood test, a day-2/3 FSH level, and an antral follicle count — an ultrasound that counts the small resting follicles visible at the start of a cycle.
Beyond age, the egg supply can fall faster because of smoking, surgery on the ovaries, endometriosis, and cancer treatment such as chemotherapy. PCOS is different: women with PCOS usually have more eggs in reserve, not fewer, even when they do not ovulate regularly.
What "low reserve" changes
A lower reserve mainly affects time and how you respond to treatment. Fewer eggs are likely to be collected in an IVF cycle, and waiting costs you more than it would with a higher reserve. It does not automatically mean poor egg quality — age is the stronger driver of quality — and women with low reserve do conceive, both naturally and with treatment.
What follows a low result
Usually a more direct talk about timing: starting effective treatment sooner, adjusting the IVF plan to the expected response, or considering egg freezing if pregnancy is planned for later.
When to see a gynaecologist
Worth a visit if you're planning pregnancy and want fertility guidance, have been trying to conceive for 6–12 months without success, or have a history of hormonal or menstrual irregularities.