What would happen if you were pregnant and had cancer?

It depends what type of cancer. Usually, Breast cancer is the most common type appearing during pregnancy. The reason why is the cancer is fueled by estrogen and progesterone, and during pregnancy those hormones are super high so it grows fast. The oncologist will then discuss the risks and benefits with the patient and it is up to her whether to abort and have treatment or wait to deliver and risk the cancer growing and spreading. If it was something slower growing like skin cancer most likely it would be prudent to wait to have any radiation or chemo. However surgery might still be an option while pregnant.