Abstract: The monitor comprises beads threaded onto a loop of elastic cord. The cord is formed into a loop by tying the two ends of a length of cord in a reef-knot. The daily progress of the menstrual cycle is marked by pulling the knot through holes in the beads. The beads are coloured to indicate the sectors of the cycle—red for menstruation, pink for fertility, black for PMS. The knot is matched to the elastic cord and to the hole in the bead so that the knot is tight in the hole, but the pull-through force is smaller than the elastic limit of the cord.