It depends on how advanced the cancer is and whether they need to take drastic measures or not. I know a woman who was diagnosed with cervical cancer during her first pregnancy and she has gone on to become pregnant two other times. It mainly depends on the treatment you receive. Chemotherapy can make you sterile but there are other treatments that don't- it just depends on how bad it has spread and how aggressive the cancer is.
You need to discuss this with your oncologist.