On day 21 of clomid (clomiphene) you need to do a blood test. what are they testing for?

Progesterone: A Progesterone blood test checks that you're ovulating (releasing one or more eggs) after taking clomid (clomiphene). After you ovulate, the follicle that contained an egg becomes a corpus luteum which makes progesterone.

Ovulation: After taking Clomid (clomiphene) we can see if you ovulated by checking a Progesterone blood level about a week after we guess you ovulated (14+7 = 21). If the prog level is >3ng/ml then you ovulated. If less than that then you may need a higher dose of Clomid (clomiphene) or time to move on to stronger medications.