Male fetal DNA is found in the brains of mothers.

http://www.latimes.com/health/boostershots/la-heb-women-brai...

I could imagine some sort of organ senses - child has certain hormones (eg testosterone) in salive, and certain receptors on the nipple pick up on these hormones and then tailor the milk.

Note that I have no basis for this claim, but one of the many ways the body communicates is through hormones, so it certainly wouldn't be outside the realms of possibility...

Bluetooth LE / iBeacons