I really liked Briggsy's Girl's advice.

A lot of people at 18 aren't ready to lock in to a career path and in my opinion, having children young can be a fantastic thing, (18 is an adult and not a child) but she really needs to realise how hard single parenting can be and the impact that could be had on family and friends.

Maybe you could tell her your true feelings on the subject but that you will be supportive of her in whatever direction she chooses? I know I would find it hard to express my feelings on the topic to my daughter without getting emotional, but like you say, she may get defensive again and it will make things harder for you.

I know in the past I relied too much on my Mother's opinions and thoughts and as such our relationship has stayed as 'Mother and child' and not evolved in to a Mother/Daughter respectful friendship, as I see my other friends have and I wish for. I can only guess, but I think it's because my mother hasn't let go at her end and has never wanted us to fail or 'stuff up', but I'm afraid it's inevitable in life and sometimes you just have to hope the work you've done in the past has paid off.. ( Insert father/daughter where applicable )