Semi-Open adoption - when would you want to know if your child's adoptive mother preg

I was wanting some advice. I am an adoptive mother and want a birth mother's opinion on how best to address a pregnancy with my son's birth mother. My DH and I recently discovere when seeing a specialist for something else, that we were misdiagnosed, and should be able to have children with little to no medical help. We had a second consultation with an RE and she was even more postive about our situation than the Dr. that discovered the mistake. We aren't going to try till December because I am taking a medicine for a condition I have that will help lower my chances of miscarriage. So my question is if we do become pregnant at what point should be tell our son's birthparents? We do letters and pictures every six months and at mothers day and Christmas.