...Although we don't really know how long a Klingon pregnancy takes. We got a "quarterbreed" pregnancy in both TNG (Alexander Rodzhenko is 3/4 Klingon) and VOY (Miral Paris is 1/4 Klingon). The former supposedly spanned SD 42901-43205, that is, just 300 stardates or three-four months - but only assuming that Alexander correctly quotes his own birthday in "New Ground", and that his conception took place during "The Emissary" and not during the previous liaison between Worf and K'Ehleyr. The latter spans at least SD 54452-54973, or 500 stardates or six months - but the starting date is not that of conception, it's that of Torres discovering she is pregnant by an estimated seven weeks, so it's a pregnancy of a bit less than eight months.
Since Alexander is theoretically closer to full Klingon biologically, it seems safe (with the above caveats!) to assume that Lursa's child was born less than 11 months after "Firstborn". Unless these short pregnancies are typical of hybrids and shorter than those of either participating species.
Timo Saloniemi