I am sure that you are sensitive in the general sense, and that you never complain in front of them about how difficult it is to be pregnant or to have to find a babysitter. I assume that such things go without saying for you. I interpret your question as how to avoid causing them hurt just by existing. You feel that you are a reminder to them of their situation, and that you cause them grief. Maybe you hesitate to visit them because you don’t want to “stick it in their face” and you only call when your baby is sleeping. That is respectful and kind, but you won’t be able to maintain a stinted relationship.
I find myself giving the same advice as in most interpersonal situations. You are not the cause. People in such a situation may feel jealous or hurt, it is natural. But it is they who must learn to deal with their emotions. They are responsible for their emotions and reactions. I doubt that they want to lose a friendship with you, and if they are smart they will do what it takes to get in control of their feelings. Your job is to not withdraw your friendship and to never complain to them about a situation they wish for. If you are doing everything right, then the rest is not up to you.
I knew someone who had no children for a number of years, and she was not able to remain friends with anyone who had children. They all felt uncomfortable around her and one by one began to have less to do with her. B”H now she has children, but I don’t know if any of the old friends returned. She should have learned to deal with her situation, and should have sought help. She suffered more because she did not. I don’t know of anyone who was insensitive (of course, sometimes people slip, but in general not). The loss of friendship was entirely her own fault. (Some people were even afraid of Eyin hara from her)
P.S. areivim: Notice that I am not saying “geit mir nicht un”. I am saying that it is the offended party who should seek help if they cannot deal with a sensitive person like SJS.