hi
firstly, i assume you have reported the damp problems to the HA? What did they say about it? They have a responsibility to resolve any repair issues but often, what people believe is damp, is actually condensation and is caused by inadequate heating & ventillation. you can do a lot to help this by keeping the property warm and by letting out steam in the bathroom & kitchen when using them.if the damp is black spots on the walls & ceillings, that is probably condensation. If the damp is caused by a leak in the roof or rising up from the ground, it is possibly a damp problem.
As for their legal responsibility, they are not legally obliged to move you because you are pregnant as they housed you adequately when you moved into the property, and they are not responsible for your circs changing. You need to address your rent arrears as this will be a barrier to you getting another property - you are in breach of your tenancy agreement if you are in arrears so they are not obliged to offer you somewhere else.
Saying that, most HA's will be understanding of your situation. However, waiting lists are very long and there will always be a wait involved, his is nobodies fault, there is just more demand for housing than supply. Some HA's will let you move if you have a small amount of arrears as long as you maintain a payment agreement and continue to pay the arrears after you move. If you haven't reported the damp, you need to do this and get a surveyor around to look at.
Your other option is to exchange with another HA or Local Authority tenant, if you can find someone who would want to swap properties with you, you would need both landlords permission to do this and you would have to clear your rent arrears before an exchange could be agreed.
Good Luck