Recently, PLL writer Andy Reaser weighed in on the mystery. A fan asked him, "When is Melissa gonna have her baby?! It's been one LONG pregnancy!"
Andy replied, "Tv time is different than real life time."
And to be fair, Andy is totally right. Pretty Little Liars is in its third season on air, but only a year has passed in the show's universe. That said, Melissa has been pregnant since the second half of Season 1, which means even the "TV time" excuse is wearing awfully thin.