has god made a mistake by giving mensturation to women

I don't think it was a mistake. I mean, it keeps everything healthy and fresh for a new baby, if needed. And, ovulating pretty much ensures that the egg is healthy and new, too. Can you imagine if the lining of the uterus was never shed? It wouldn't be healthy for the baby. Up until modern day, most women were pregnant for most of their adult life, so they rarely every had a period, anyway.