A conception calculator (410) comprises a calendar slide (414) and an indicator slide (412) that are relatively rotatable. The calendar slide is divided into at least three, but preferably less than twelve, month-segments (424A-C). The indicator slide is marked with, or is set out to be marked with, a fixed start-of-last-period indicator (462), which a user can align with that one of the divisions of a scale on the calendar slide corresponding to the date of the start of her last period. The indicator slide is set out to be marked with a fixed fertility indicator (464) which is angularly offset from the start-of-last-period indicator and which indicates on the calendar scale a sequence of days-of-the-month when the user can expect to be fertile. The angular offset of the indicators and the number of days in the sequence indicated by the fertility indicator are both preferably arranged to be determined in dependence upon the values of two numeric parameters related to the user's menstrual cycle history (for example her shortest cycle length and her variation in her cycle lengths in recent months) so as to personalise the calculator. The calculator therefore takes account of the user's menstrual history yet employs only two slides, as a result of which the calculator is of simple and inexpensive construction and, once personalised, is easy to use, requiring only one movement of the two relatively-movable slides in order to set the start date of the last period.
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