After ovulation, the empty follicle transforms into a temporary hormone factory called the corpus luteum, which produces progesterone. Progesterone changes the uterine lining from "growing" to "receptive" — ready for an embryo to implant — and keeps it stable through early pregnancy.
Why it's given in fertility treatment
In IVF and often after IUI, progesterone is prescribed as vaginal gels or pessaries, tablets, or injections — called "luteal support." IVF cycles in particular need this support because the egg retrieval process affects the corpus luteum's natural production.
On lab reports
A progesterone blood test in the second half of a natural cycle can confirm that ovulation actually happened — one of the simplest checks in a fertility evaluation.