I wonder how much of this is because the mother completely failed to protect her vulnerable daughter
I'm very sceptical about the mother's role in this.
The family have already failed to protect their child
Why is the mother getting blamed?
If a child gets sexually abused, is it the mother's fault?
We don't know what the care arrangements are but it's very likely that the mum isn't providing 24/7 care. She may well go to a care facility or even college. There's a good chance she has carers in the house. She might even have a degree of independence when she does not require supervision.
why has the grandmother allowed the poor girl to get to 22 weeks? She must have known her periods had stopped
You are assuming that this young woman is incapable of meeting her own menstrual needs and that there are no other carers involved.
You are also assuming that she didn't know. She may have known for a while now but it has taken time to get through appointments, social services and the court.
It shows how the notion of having a mental age is flawed and misleading.
I trust that the courts have made the best decision in this young woman's best interests. It's a heartbreaking story and they will clearly have to review her care requirements.