i had to tell my boss pretty much straight away, because of the job i am in (with a weight loss org). but unless your job puts you at high risk, or there are other factors / conditins which may effect employment (such as i am supposed to maintain a certain weight - obviously not possible while pregnant); you are only legally required to inform your employer 10 weeks before you intend taking maternity leave. although i think it may get a bit obvious by then anyway. get intouch with your HR department and find out you rights and obligations.