After the birth of your baby and the placenta, your body will produce Prolactin. This is the Hormone which tells your brain to start producing milk. Most women will produce milk from day two to day six. The day your milk comes in can depend on the type of birth you have had. Some studies suggest that milk production is delayed in women who have a caesarean section. A delay can also be caused if a woman is on medication such as Insulin to control blood sugar levels.
Ref; Expecting everything you need to know about pregnancy labour and birth 2004.
McGrail A, Metland D
MIDIRS Informed choice leaflet; No.7 breastfeeding or bottle feeding