Well, just as a practial matter, if the woman has cancer, it’s likely impossible for her to have a child while being treated. Radiation would kill a fetus, as would chemotherapy or the newer drugs they use to suppress fast-dividing cells. If the treatment didn’t kill the fetus, but damaged it severely, the parents would be guilty of purposely harming their child by conceiving it during that time… pre-natal child abuse! This assumes they knew of the cancer before conception.
If the man has cancer, he may not produce enough healthy sperm to conceive a child. Or his sperm may be damaged by the treatment, and his child harmed in that way.
There’s a difference between trusting God and making rash, foolhardy decisions and then wanting Him to bail you out. No couple where one is facing death should be having children. If the sick person gets healed, then the couple can pursue parenthood. But parenthood is not a right, and the needs and rights of the child must be foremost in considerations of whether to become parents.