I don't kill Sims.  If I get a baby I don't like I simply control+click and delete the baby.  Sure the mother walks around for awhile thinking there is a baby there, but it is fleeting.  Then just try for another baby. 

Are you making your females all really muscular, or perhaps ugly?  Do they have masculine traits such as angler, handy and criminal or super athletic?  I am just wondering if traits may be a contributing factor in sex determination.    Have you tried making a female that is absent minded, hopeless romantic, natural cook, friendly and family oriented?  I wonder what sex her babies would be?   Oh wait...didn't you say you went to China?  If you got Preggo in China, don't hold your breath for a girl.