It amounts to forced abortion. I think forced abortion is a bad thing.
There are very few girls who could pull off a secret abortion (even with the help of an adult male abuser). However, if the doctrine of the immaturity of all girls under 18 stands, it would be more common for parents to force an abortion on a daughter. It happened in Italy.
I think abortion is a bad thing, forced or otherwise. I also think that parents being involved in their children's lives is a good thing. Not allowing their involvement in a medical procedure fraught with not a few risks, emotional and otherwise, is both at odds with other standards in our healthcare system and at odds with the idea of parents being able to raise their children in the manner they best see fit.
So, even with the risk of forced abortion which you posit, I will still support the idea of parental notification in the belief (and hope) that parents will be far better caretakers of their daughter's wellbeing than Planned Parenthood or the National Education Association.