singular they - Correct pronoun for "baby"

Stumbled along the use of the feminine pronoun for "baby". Previously, I've used it and singular they when the sex of the baby is unknown.

• Is it correct to use the feminine pronoun (she) for "baby" when you don't know if it is a boy or a girl?


• Can somebody please explain that to me?


• Here's an example: "At about 6 months, when your baby can sit up, she'll probably be too big to be bathed in an infant tub" (http://www.consumerreports.org/cro/baby-bathtubs/buying-guide.htm)