What is the role of prolactin in pregnancy?

Role of Prolactin in Pregnancy

Prolactin plays a critical role in pregnancy by preparing the mammary glands for lactation, with levels increasing 10-20 times above normal by the end of gestation, though its milk-producing effects are blocked by high progesterone until delivery. 1

Physiological Changes in Prolactin During Pregnancy

• Prolactin secretion from the anterior pituitary increases progressively throughout pregnancy:

  • Levels range from approximately 6 ng/ml in early pregnancy to 210 ng/ml near term 2
  • By late pregnancy, prolactin reaches 10-20 times normal non-pregnant levels 1
  • This increase prepares the mammary tissue for lactation after delivery

• Despite high prolactin levels during pregnancy, milk production is inhibited by:

  • Elevated progesterone levels that block prolactin's effects on the mammary gland 1
  • Only after progesterone and estrogen levels fall at delivery does copious milk production begin

Key Functions of Prolactin During Pregnancy

Mammary Gland Development

• Promotes growth and differentiation of mammary tissue during pregnancy 3
• Contributes to the increase in size and number of breast ducts and lobules 3
• Stimulates development of secretory capacity in mammary epithelial cells

Molecular Actions in Breast Tissue

• Stabilizes and promotes transcription of casein mRNA 1
• May stimulate synthesis of alpha-lactalbumin, a regulatory protein in lactose synthesis 1
• Increases lipoprotein lipase activity in the mammary gland 1

Metabolic Adaptations

• Helps drive maternal metabolic adaptations to meet the high energy demands of pregnancy 4
• Affects tissues involved in metabolism including pancreas, liver, and adipose tissue 4
• Contributes to energy homeostasis during the transition from pregnancy to lactation

Prolactin Forms and Activity

• Exists in three heterogenic forms with varying biological activity 1:
  • Monomer (23 kDa) - principal biologically active form
  • During pregnancy, the proportion of monomer increases relative to other forms
  • During lactation, the mammary gland selectively takes up the monomer form

Postpartum Role

• After delivery, when progesterone levels fall, prolactin can exert its full lactogenic effect 1
• Minimal hormonal requirements for normal lactation include prolactin, insulin, and hydrocortisone 1
• Suckling stimulates continued prolactin release, maintaining milk production 1
• Prolactin levels remain elevated (around 1000 mu/l) during the first 15 months of lactation 5

Clinical Considerations

• Prolactin levels alone do not appear to be a valuable guide to maternal or fetal well-being 2
• Some evidence suggests maternal and fetal prolactin may not play an obligatory role in pregnancy maintenance or fetal growth 6
• Breastfeeding stimulates oxytocin release, which may provide protection against postpartum depression 7

Potential Pathological Implications

• Abnormal prolactin levels have been observed in some complicated pregnancies:
  • Approximately 14% of samples from patients with threatened abortion show elevated levels 2
  • 7% of samples from hypertensive pregnant patients show levels above the normal range 2
  • Lower prolactin levels were associated with low urinary estrogen excretion 2

While prolactin is primarily known for its role in lactation, its significant increase during pregnancy highlights its importance in preparing the maternal body for breastfeeding and potentially contributing to metabolic adaptations necessary for successful pregnancy outcomes.