Humans and primates, are quite different. They have a menstrual cycle. In this case, females are sexually receptive at any time, but only fertile when an ovum is released from an ovary. In this case, the endometrium (if not needed for an fertilised egg) is discarded. The endometrium is shed, and takes with it a certain amount of blood. In this system, eggs are released from the ovaries mostly in the middle of the cycle, away from the menstrual period. This ovulation is 'concealed', meaning, it is not obvious when it occurs. This process, so it is thought, tends to keep the male and female together, which is unusual in mammals with the oestrous cycle.