Natural Limits of Pregnancy Testing in Relation to the Expected Menstrual Period

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Abstract
The practical reference point for the detection of pregnancy is the first day of a woman's missed period, ie, the day on which she expects her next period to begin. Unless a woman carries out special tests for ovulation, her only basis for predicting the onset of her next menses is her usual cycle length. Most women are able to report their usual cycle length, despite the natural variability from cycle to cycle. This variability in cycle length is due largely to variation in the number of days from the beginning of menses to ovulation.1