Menstruation
Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).
Dog behavior
Female dogs have an estrous cycle that is nonseasonal and monestrus, i.e. there is only one estrus per estrous cycle. The interval between one estrus and another is, on average, seven months, however, this may range between 4 and 12 months. This interestrous period is not influenced by the photoperiod or pregnancy. The average duration of estrus is 9 days with spontaneous ovulation usually about 3 days after the onset of estrus.