All my 4-year-old son has been talking is that Mommy and Daddy are having a baby and what to get for…

All my 4-year-old son has been talking is that Mommy and Daddy are having a baby and what to get for the baby and he hopes the baby won't poop on his bed....... I so wish we could give him the baby sister he wants so desperately but no matter what we try it seems impossible to get pregnant....... I feel so bad for my little bug.....