Parentlock

Especially because of Season Three's depiction of heavily pregnant Mary in "His Last Vow," fans who ship Johnlock are more likely to consider the inclusion of a Watson baby within the parentlock narrative of Sherlock and John.

The relevant tags here were very similar to LJ, if phrased slightly differently: e.g., parentlock (portmanteau of parent + Sherlock), military John, genderswap, mpreg.

All of these AU parentlock stories or series become "alternate" because of a change in the Sherlock-John relationship and the inclusion of a child who becomes theirs.