I have been through the same situation. I am going to take the X ray even though I am pregnant and my doctor advised that I wait till later after I give birth, but after long research and investigation with most clinics in Canada, I found that it is safe to take the X ray as long as you are going to wear the apron (abdominal shielding) to protect the baby.
I decided not to wait till I give birth and delay the process of my application because I am the main applicant in my application. I am a mom in the first place and my baby comes first of course, so what I am saying is that I checked with highly professional doctors in more than one city in Canada and they assured me that my baby will get no risk as the chest X ray has minimal radiation than other kinds of X ray. Furthermore, it is only once that you are going to do the X ray during pregnancy, so, no worries. Just make sure to tell them that you are pregnant when you are to take the X ray so they give you the apron.If not convinced, just wait till you give birth and then go a head with the process of your application.
Again I would like to thank those who responded to me when I posted a similar question in the forum.
Good luck for both of us!!
Maple