Typical baseline serum prolactin levels in women	Prolactin Levels	References
Not pregnant, not lactating	< 25 ng/mL	Walker p. 65
Pregnant, at term	200 ng/mL	Walker p. 65
Lactating, 7 days postpartum	100 ng/mL	Riordan p. 76
Lactating, 3 months postpartum	100 ng/mL	Walker p. 65
Lactating, menstruation not started before 180 days	110 ng/mL	Riordan p. 76
Lactating, menstruation started before 180 days	70 ng/mL	Riordan p. 76
Lactating, 6 months postpartum	50 ng/mL	Riordan p. 76

Note: Riordan J. Breastfeeding and Human Lactation (2005, p. 76):

• "Plasma prolactin levels increase the most in the immediate postpartum period but rise and fall in proportion to the frequency, intensity, and duration of nipple stimulation."
• " Prolactin concentration in blood doubles in response to suckling and peaks approximately 45 minutes after the beginning of a breastfeeding session (Noel, Suh, and Frantz, 1974)."
• "During the first week after birth, prolactin levels in breastfeeding women fall about 50 percent. If a mother does not breastfeed, prolactin levels usually reach nonpregnant levels by seven days postpartum (Tyson et al., 1972)."
• Prolactin levels "follow a circadian rhythm: levels during the night (sleep) are higher than during the day."
• Prolactin levels "decline slowly over the course of lactation (Battin et al., 1985; Cox, Owens, & Hartmann, 1996) but remain elevated for as long as the mother breastfeeds, even if she breastfeeds for years (Stallings et al., 1996)."
• Prolactin levels "rise with suckling: the more feedings, the higher the level of serum prolactin. More than eight breastfeedings per 24 hours prevents decline of the concentration of prolactin before the next breastfeeding (Cox, Owens, & Hartmann, 1996; Tay 1996)."

References:

1. Riordan J. Breastfeeding and Human Lactation, 3rd ed. Boston and London: Jones and Bartlett, 2005: 75-77.
2. Walker M. Breastfeeding Management for the Clinician: Using the Evidence. Boston: Jones and Bartlett, 2006: 63-66.
3. Noel GL, Suh HK, Frantz AG. Prolactin release during nursing and breast stimulation in postpartum and nonpostpartum subjects. J Clin Endocrinol Metab. 1974 Mar;38(3):413-23.
4. Tyson JE, Hwang P, Guyda H, Friesen HG. Studies of prolactin secretion in human pregnancy. Am J Obstet Gynecol. 1972 May 1;113(1):14-20.
5. Stallings JF, Worthman CM, Panter-Brick C, Coates RJ. Prolactin response to suckling and maintenance of postpartum amenorrhea among intensively breastfeeding Nepali women. Endocr Res. 1996 Feb;22(1):1-28.
6. Cox DB, Owens RA, Hartmann PE. Blood and milk prolactin and the rate of milk synthesis in women. Exp Physiol. 1996 Nov;81(6):1007-20.