Can we delay a pregnant worker’s start date rather than grant leave?

Q: We hired a new employee who told us after she was hired that she is pregnant with a due date about six weeks after the start date we offered (and which she accepted). Can we change her start date to several weeks after her baby is born, or would that be considered discrimination against a pregnant woman? Also, how do we go about writing a maternity leave policy that is legally sound (do we need a lawyer for this, or is there an easier and less expensive way)?  – Lilly, California