That is one of those decisions that is up to you, not your family. They don't get a vote! I think you are smart to get it in writing.
A very good friend of mine, we call each other sisters from other mothers and we are as close as is humanly possible, lost her 25 year old daughter to cancer. She had two children, then the cancer diagnoses and she had treatment She was advised not to have any more children and the doctor was worried that the cancer would come back, but that was because of the type/location of the cancer and her general health was the part of the reason he suggested no more babies. The cancer did come back and she started treatment, then she got pregnant again. She continued the pregnancy and treatment for the cancer stopped. When the baby was just a few months old, she died. Her funeral was held on what would have been her 26 birthday. At the time, from the way she talked, most people (including her own mother) thought that she really did not want to continue the pregnancy, but her husband (who was a druggie) overruled her wishes. My friend is raising the children as the druggie father abandoned them not too long after she died, and she loves her grandkids including the youngest, but I don't think she will ever get over losing her daughter. I don't know if the ultimate outcome would have been any different, but the point is that it was her decision and it was taken from her. Her husband's reason was the church's standpoint on abortion. Not that he ever actually attended church, but that was the reason he insisted on her continuing the pregnancy.