In contrast with a male zav who is suffering from a venereal disease, a female zava is a woman who experiences a flow of menstrual-type blood during a time of the month when she is not due to experience menstrual bleeding. According to Biblical law, when a woman experiences her menstrual cycle she is a nidda whether she bleeds only once or many times over a period of seven days. At the end of seven days she immerses in a mikvah and is rendered ritually pure. The eleven days that follow are called yemei ziva. If she experiences vaginal bleeding during that time she is rendered a zava.
The laws of zava differ from those of a nidda. If a zava experiences bleeding just once or twice during that period, she is deemed a zava ketana who will “keep watch a day for a day” – she must check that she is free of bleeding one day for each day that she bled. After experiencing bleeding on a third day, however, the woman is considered a zava gedola and is obligated to wait a full seven days without bleeding. At that time she can immerse in a mikvah and she will be permitted to her husband. The next day she must bring a sacrifice as part of her purification process, which will allow her to enter the Temple and consume sacrifices (see 15:25-29).
The Mishna discusses a situation where a woman in childbirth begins to experience bleeding. If it occurs on days that we anticipate that she will be a nidda, she is rendered a nidda. If, however, it occurs on days that we anticipate that she will be a zava, she will not become a zava, since she will only get that status if the bleeding is from her and not if it is caused by the emerging fetus.