It happens. This has happened before to childbearing age women. All cancer doctors have experienced this. Depending on the exposure of radiation and the trimester of pregnancy frank discussions of risk to the baby take place. If it happens while taking treatment as you state and the belly is not the area of treatment there may be ways to shield and protect the baby. You can not directly radiate the baby.
Try to avoid this. While enjoying a sexual life during radiotherapy is perfectly ok, getting pregnant is to be avoided. The exposure of the embryo and fetus can lead to developmental abnormalities, spontaneous abortion, and tough moral choices. There are consequences and risks during each trimester of pregnancy. The best advice is to try to not risk pregnancy during cancer treatment.
Prevention is key. For child bearing age female who needs irradiation, the most important thing is to prevent getting pregnant before radiation starts. If it does happen, the foetus is most susceptable in the 1st trimester and potentially mutation or deformity can occur. Extensive counseling is necessary to acertain the dose and risk and decision needs to be made on ? Abortion.