A weekly test is an awful idea, but it says a lot about Umbridge's lack of understanding how female anatomy works. From her letter to Kerr this is just a badly thought out excuse to check Sinistra isn't having sex, on a weekly basis.
However, won't it reflect badly on Kerr later if he carries her orders out? I can't imagine other Healers buying any need for a weekly, rather than monthly test. Of course, I might be seriously overestimating how much the wizarding world understands about sexual reproduction.