When I said "your cervix isn't really that 'closed' at that time," I meant when you are in the middle of doing IVF, not when you are pregnant.
As GA says, no doctor would do IVF on a pregnant woman. Your cervix isn't really that "closed" at that time. They slip a small catheter into the os, the opening at the end of the cervix. By then, you've had so many hormones that your os is letting out a nice string of clear, copious mucus anyway. In other words, it is not like a tight seal or a thick mucus plug, such as happens during pregnancy.
If you are already pregnant, compromising the seal of the cervix could cause a miscarriage. However, I would think that any IVF doctor you use would ensure that you are not already pregnant before performing the procedure.