I won't elaborate on the fact that Chinese families have a preference for sons.
Also most of you know that because of widely practised sex-selective abortions (aborting the female fetuses), the in-pregnancy screening for gender is outlawed in China.
So, what options does it leave to parents who still want to be "in control" of their only baby's sex?
It seems that besides paying extra-money to perform illegal screening, and using some dubious baby gender predictors - Chinese parents can look for hints from a doctor who makes the ultrasound.
According to a theory, your ultrasound examiner will be likely to impart subtle hints. For example, "take care of the baby" means that you have a boy, while "boys and girls are all the same" implies that you'd better prepare for a girl.