I have been offered a job in a large cooperative company ( my first job in Spain). I have arranged to sign the contract later this month then start 1st September. I am pregnant and would like to tell them before signing the contract. Do you think it will jeopardise the job offer if I do this?  How will it be received if I keep quiet?

Morally if you kept it quiet then you have to live with yourself.

You may/not know that once you have a permanant contract (fijo/indefinido), you will get lots of maternaty leave and paid while you are away.  But the co cannot hire someone else in full time to do your job whilst you are off as they have to leave the job open for you.  Also they will have to hire and pay a temp while you are away. (Social security payments by the co are crippling for any type of employee as they do have a good payout system if you are sick/need support for a while). 

I would be surprized if you are being offered a fijo from day one (although I have heard of it).
Usually you get a temorada for 3 months, another 3 months and the co can even then go for antoher of 6 months before they have to offer you a full/permanant contract.

I don;t think so that you should tell these to them as i you told them might be they will reject you just to be on safe side or being selfish.If you get an job your employer have to give you leaves and payment for your maternity cause and there is nothing wrong in it because as employer that all are there duty and as employee these are your rites.So accept the job and never feel sorry as there is nothing personal its business.