What hormone decrease is most likely responsible for the transition from colostrum to mature milk in a postpartum woman, characterized by a change in breast milk from thick and yellowish to thinner and whiter?

From the Guidelines

The answer is F. Progesterone. The changes observed in milk production from thick, yellowish colostrum to thinner, whiter mature milk are primarily due to the decrease in maternal serum progesterone levels after delivery of the placenta. During pregnancy, high progesterone levels inhibit lactogenesis (milk production) [ 1 ]. After birth, the sudden drop in progesterone removes this inhibition, allowing prolactin to stimulate milk production. This transition typically occurs around 2-5 days postpartum, coinciding with the breast engorgement and milk composition changes described in the scenario. Progesterone's withdrawal is the key hormonal trigger that initiates the shift from colostrum to mature milk, while prolactin levels remain elevated to maintain milk production [ 1 ]. The other hormones listed (estrogen, oxytocin, hCG, thyroxine) do not play this specific role in the transition from colostrum to mature milk. Some studies suggest that prolactin has anxiolytic properties and may contribute to the stress-buffering effects of lactation [ 1 ], but the primary role in the transition of milk production is attributed to the decrease in progesterone levels [ 1 ]. Key points to consider include:

• The role of progesterone in inhibiting lactogenesis during pregnancy [ 1 ]
• The effect of progesterone withdrawal on initiating milk production [ 1 ]
• The timing of the transition from colostrum to mature milk, typically around 2-5 days postpartum [ 1 ]
• The distinction between the roles of progesterone and prolactin in milk production [ 1, 1 ]

From the Research

Changes in Milk Production

The observed changes in milk production, from thick and yellowish to thinner and whiter, can be attributed to the decrease in maternal serum concentration of a specific hormone.

• The initial milk produced is colostrum, which is high in density and low in volume, and is produced until about 30 hours after birth 2.
• After 30 hours, the milk production increases in volume and decreases in density, resulting in thinner and whiter milk.
• The hormone responsible for this change is likely to be progesterone, as its levels decline rapidly after birth, allowing for the onset of lactation 2, 3, 4.
• Progesterone inhibits milk synthesis during pregnancy, and its decline after birth enables the initiation of lactation 2, 3, 4.
• Other hormones, such as estrogen and prolactin, also play a role in lactation, but the decrease in progesterone is the primary trigger for the observed changes in milk production 2, 5, 6, 3, 4.

Hormonal Changes

The hormonal changes that occur during the peripartal period are complex and involve the interplay of multiple hormones.

• Progesterone levels decline rapidly after birth, while estrogen and prolactin levels increase 2, 3, 4.
• The decline in progesterone and the increase in estrogen and prolactin enable the initiation of lactation and the production of milk 2, 3, 4.
• The exact mechanisms by which these hormonal changes regulate milk production are not fully understood and require further research 3.