A Shih Tzu normally goes into heat for the first time when she is about 5 to 6 months old. Also known as being "in season," the period is marked by light to moderate bleeding, which generally lasts from 12 to 21 days. A mature female continues to go into heat approximately twice a year for the rest of her life unless she is spayed.
A female dog's cycle is divided into four stages, which mark changing levels of fertility and receptiveness to the attention of male dogs. During the first part of the cycle, proestrus, the female has a blood-tinged vaginal discharge and is attractive to male dogs, but she is usually unwilling to mate. This period lasts from four to 21 days and is followed by estrus, a 5- to 13-day period during which the female has a yellowish vaginal discharge and is most receptive to the attention of male dogs. Mating occurs during this time. If she does not become pregnant, the female then enters metestrus (also called diestrus) a period of about 60 to 90 days during which no mating occurs. If she is pregnant, gestation lasts from 58 to 63 days.The final stage of the cycle, anestrus, is a time of sexual and hormonal inactivity which lasts about two to three months.