Why you might see this term
Testing is often arranged about seven days before the expected next period. Day 21 applies to a 28-day cycle, not everyone.
What to keep in mind
A single result is sensitive to timing and cannot by itself grade the quality of a luteal phase or confirm pregnancy.
Sources & further reading
Prepared by ProFertility · Updated 23 October 2026.
Educational information based on the linked sources. This explanation is not an individual diagnosis or a treatment recommendation. A term appearing here does not mean the test is necessary or included in a particular kit.
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