If a woman sees blood and then her baby becomes recognizable within 24 hrs we say מעת לעת .

A woman in hiding who sees blood is only טמא from here on in.

According to ר"מ as long as a woman nurses דמיה מסולקין .

The reason the Torah writes Mazeh is to teach us that the Nosei only becomesTamei if he carries the amount Mei Nidah that is required for sprinkling.

In order to be considered a זקנה she has to miss 3 עונות .

In the end analysis each עונה is 30 days.

A girl who is too young starts seeing blood we say she is מטמאה מעת לעת only at the third time.

Once she is old enough to see her first time is מטמאה מעת לעת.