Fertility testing
Often called a 'day 21' test, it should be timed around seven days before the next expected period, so the day differs for longer cycles.
Also known as: day 21 progesterone, day 21 test, ovulation blood test
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Last reviewed: 23 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.