http://en.wikipedia.org/wiki/Prolactin
"Sucking by the baby on the nipple then promotes further prolactin release, maintaining the ability to lactate. The sucking activates mechanoreceptors in and around the nipple. These signals are carried by nerve fibres through the spinal cord to the hypothalamus, where changes in the electrical activity of neurons that regulate the pituitary gland cause increased prolactin secretion."
This translates to possible developement or worsening of pre existing gyno if Im not mistaken?
I am punching my girfriend the next time she twists my nipples when I'm on.