Name the stimulus and hormone which initiate lactation.

Prolactin is the most important hormone for the actual initiation of lactation postpartum. Milk secretion is regulated by PRL through an action on plasma-lemmal receptors of mammary gland Alveolar secretory cells.
It is ,however, the fall in estrogen and progesterone levels which gives the stimulus to lactation. Baby suckling also provides stimulus to lactation.