Sarah Down, from Topsham, begged that her daughter should be taken into care when she ran away last year to live with a 31-year-old man in his car.
But the authorities refused - and Katie, now 16, is due to give birth in a fortnight's time.
That means her baby was probably conceived around August last year - the same time that Ms Down was pleading that the teenager should go into care - and weeks before her daughter's 16th birthday, in October.
Katie had run away from home in March and had spent months sleeping in a tent or a car owned by her then boyfriend, Derek Rice.
But social services said they could not take her into care - and in September magistrates ordered that she could live with Mr Rice's parents at their home in Clyst St Mary.
Ms Down says more action should have been taken to put Katie - a former Priory High School pupil - into a care home until she reached the age of consent in October.
The 34-year old care worker says the last year has been a nightmare dealing with the stress of the problem.
She said: "I feel they could have done more and stopped a lot of things happening. She fell pregnant before she was 16 and she should have been in care.
"My feeling towards social services is that I think they are a waste of time. I would never turn to them again because there is no point.
"This is what I feared was going to happen. When I found out I was absolutely devastated. I'm upset but now I'm there for Katie and the baby. I think Devon County Council failed in their responsibility to look after her."
Last year, social services said they had no powers over Katie and could not take her into care unless the police arrested her.
They said the only way a young person could be forcibly taken out of such a situation was for an application for a 'secure order' to be successful through the courts.
The police said they could not arrest her because no offence had been reported.