You will still need to undergo a medical exam even if you are pregnant. However, certain parts of the exam may be postponed until after you give birth. In that case, your application will be assessed only after all mandatory medical requirements are met. You should discuss your options with the Panel Physician performing your medical exam.
so have a chest X-ray at some stage. This is a part of the tuberculosis control program. If you are pregnant and are concerned about having an X-ray, you have a few options
1 You can have the X-ray now, with a special protective shield covering your abdomen. This protects your baby from radiation. (This is probably safe but no
guarantee can be given, cos its not safe
2 You can do the other parts of your medical examination now and have the X-ray after the birth of your baby.
So there are 2 options, but your wife will do it, natin to fear about
source
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=987&top=4