Dr. Mary Wingfield:

It is a good question. There is a medical reason for that. Unless any of has been pregnant we do not know whether we will have a complicated or uncomplicated pregnancy. Some women develop complications during pregnancy, and they are more common in a first pregnancy, such as high blood pressure and needing to be delivered early. I have seen cases where women on their first pregnancy have ended up with a major haemorrhage and lost their uterus, so they cannot carry another pregnancy again. Again, it comes back to protecting the surrogate. If one knows the surrogate has had uncomplicated pregnancies previously, it is very likely she will have an uncomplicated pregnancy again. One does not want her to have a very complicated first pregnancy that affects her chances of having a healthy pregnancy for herself in the future if she does not have any children.