It kind of depends . . . the CD 21 is what is standard for "7 DPO." If you haven't O'd yet, then I'd wait until 7 DPO, if you O early, you might want to try and go early, if you O right on cue on CD 14, then going on 8 DPO is fine, but I wouldn't push it much later than that - you'd be better off going a few days early (like 5 or 6 DPO).

They like to check progesterone at the "peak" post-O, which is supposed to happen around 7 DPO. Hope that makes sense!

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