I'm not a mum or even female, so I hope you don't mind asking advice here on behalf of my wife. I'm assuming people on this forum will have more experience of this issue than on my usual haunts.
My wife and I would like to start trying for a baby soon, but we have an employment issue which is hanging over us a bit. My wife has been in a job she likes for the last 18 months on 3 month contracts that get constantly renewed. The organisation she works for has been promising to make her permanent and is about to start recruiting for her permanent position. We thought, great, that's the maternity leave sorted and we can get on with things 
however today she got promoted which is both 
as it's £10k more and 
as the promoted role is still on the 3 month contract.
My question is, does anyone know there a length of time served where she accumulates the same rights as a permanent employee even though she is on a 3 month contract? She has 18 months of being employed by this organisation already, and countless verbal pledges to make her permanent.
I know if she get's pregnant we could claim SMP no problem, but we've been aiming to get on the occupational scheme and allow her to return to work after maternity. I'm thinking that with this 3 month contract, they could deny the occupational scheme and any return to work.
If anyone has any advice on this it would be useful, my wife is in the weird position of being upset about being promoted, and feels unable to turn it down and apply for the lower position.