Dogs do not really have a menstrual cycle but they have oestrous (being in heat) instead, twice a year and from around 6 months approx( differs in different breeds and in individuals). There is a saying that " Ovulation and oestrus go together but menstruation and ovulation do not". That is precisely why the oestrous cycling and the menstrual cycling are very different. ///It is during oestrous( estrous) that canines ovulate and therefore become fertile and can conceive BUT human females cannot normally conceive during their menses . ( though there are some ladies who do, there are always exceptions to any rules!!) The vast majority of( normal )human females ovulate 14 days BEFORE their next period and when no pregnancy occurs the whole hormonal reproductive system "tells" the womb lining to break down as there is no baby to use it and the whole process starts again every month from approx 13 to 50 years of age. . So you can see how very different the menstrual and the oestrous cycles are. The oestrous cycle in canines has many different stages. /// Plus humans can mate at any time but dogs can only mate at certain times during oestrus
Usually at 6-9 months of age but should not be bred in this time for she is very young.
First of all its not called a period its called a heat. When the shitzu is generally over one year of age is when the first heat is expected . It is unknown exactly when the first heat will come but it is usually (from my experience) between the months of april and june. Depending on the dog. Don't try to make your animal come into heat that is NOT advisable.