Progesterone
The hormone released after ovulation that prepares the womb lining.
Progesterone is produced mainly by the corpus luteum, the structure left behind in the ovary after an egg is released, and later by the placenta in pregnancy. It prepares and maintains the lining of the womb.
It matters because its rise is evidence that ovulation has happened, which is why it is measured in fertility assessment rather than as a general health marker. It also acts on the brain, which is part of why mood can shift across the cycle.
In practice timing is everything: measured in the wrong week the result is meaningless, so it is taken around seven days before an expected period.