The second principle of breastfeeding:
The supply and demand system
Your baby's suckling sends a message to your brain to say how much milk she needs. If your baby is hungrier today, she suckles more. In response, your brain sends out more milk-making hormones telling your breast to make more milk. It is a supply and demand system. If your baby is separated from you, or taken off your breast too soon or poorly attached, she cannot stimulate the amount of milk she needs. If your baby's stomach is filled with other fluids, she will not suckle so vigorously. Giving your baby a dummy, especially in the early days, may stop her stimulating your breast often enough.
Breast-milk contains a substance called 'inhibitor'. If milk is not removed from the breast, the inhibitor makes your body reduce milk production. The 'inhibitor' is there to shut down the milk-making system if the baby does not feed. So you can understand that restricting breastfeeding and not removing the milk will reduce the supply. Many women believe that their bodies cannot make enough milk and some health professionals mistakenly tell them this. In fact most women can produce far more milk than their babies need. All women are designed to feed twins, so producing enough for one is easy if you respond to your baby's need to suckle. When a baby wants to suckle frequently, she is ordering the milk-making system to increase production; within a few days her orders are fulfilled. Supplementary feeds interrupt the baby's clear message to her mother's body.