This is the colloquial name for a test known more formally as the AMH test (anti-mullerian hormone test). This hormone is secreted by egg sacs in a woman’s ovary. The test is a combination of a blood test with a pelvic ultrasound during the menstrual cycle, and reveals how much hormone is being produced and therefore how many egg sacs there are. It was developed about ten years ago as a tool for IVF treatments.
As women are delaying having children, they find themselves in a situation where they don’t want to miss out on getting pregnant but are unsure how much time they have. Thus, the test which was a specialist medical tool is becoming a mainstream offering to women who want to plan their lives.
The only problem is that it is not the whole answer. The test reveals the number of eggs but not the quality of the eggs. Fertility is dependent on a range of factors, the presence of the egg being just one of them. The only way for a woman to know for sure that she is fertile is to get pregnant.