Only very occasionally has one of our females had a litter after 15 days and very rarely do the young survive if they are born after the 17th day. If it is the 18th day, your female is obviously pregnant and probably in discomfort, then it is time to take her to a vet. In the experience of our friends, inducing the birth is rarely successful, a cesarian is the better option. The babies are not often saved and the Mum too stressed to raise them if they do survive. Better to concentrate on saving your female.
Problems are very rare, though.
We have only had it happen once or twice in nearly six years of hamster breeding and over a hundred litters. Hamsters on the whole make great parents. You will know that the litter has arrived by the sound of squeaking. There may be blood on the bedding and Mum will have exposed teats caused by the babies suckling. Most mother hamsters do not appreciate any disturbance.