Personally I like S/HE pronounced she or HER/M pronounced herm. They sound sort of feminine, but are economical in print and less clumsy than "he or she", etc.

However, if you already know the sex (please note I wrote sex rather than gender...when I grew up gender was only used as a grammatical device) of the eventual child, why not use that appropriate term? In any case, congratulations to your family.---John