Like breastfeeding, pumping is a learned skill.
Most mothers are able to express only a few drops of milk when they first try a breast pump.
With practice and knowledge of how the breast works, the mother will soon become more efficient at pumping. Doing this can cause discomfort and anxiety and actually discourage the let-down response. Understanding how the let-down works makes understanding pumping easier. The let-down reflex releases milk from the back of the breast into the ducts to make it available to baby or the pump. As the nerves in the nipple and areola are stimulated by baby's sucking or the action of the pump, the pituitary gland receives a signal to release prolactin and oxytocin into mother's blood stream.