Uterine contractions during childbirth and the flow of milk from the mammary glands are stimulated by:

Positive Feedback: Meaning that the stimulation causes more of the hormone to either produce milk or cause uterine contractions during childbirth to thus speed up the process.

If it were negative feedback it would be an on-off process like the release of insulin and glucagon: insulin when blood sugar is high to lower it, glucagon to heighten the blood sugar when it gets low. And since this isn't an on and off again process, the answer to your question has to be positive feedback