human placental lactogen

hPL is a peptide hormone structurally similar to growth hormone, secreted by the syncytiotrophoblast throughout pregnancy, with levels rising progressively as placental mass increases (used historically as a marker of placental function). Its primary physiologic role is to alter maternal metabolism to prioritize fetal nutrient delivery: it induces peripheral insulin resistance in the mother, decreasing maternal glucose utilization and increasing maternal lipolysis (releasing free fatty acids for maternal energy use), thereby increasing glucose and amino acid availability for the fetus—an anti-insulin, diabetogenic effect. This is a key contributor to gestational diabetes mellitus, especially in the second and third trimesters when hPL levels peak. hPL also has some lactogenic and growth-hormone-like effects, contributing to mammary gland development in preparation for lactation, though prolactin remains the dominant lactogenic hormone. Clinically, hPL was previously measured to assess placental function/fetal well-being, but this has largely been replaced by ultrasound and other biophysical assessments. Low hPL levels can be seen in placental insufficiency, while abnormal placentation (e.g., molar pregnancy) affects hormone patterns differently (hCG predominates in molar pregnancies). Understanding hPL is important for explaining why insulin requirements increase during pregnancy and why gestational diabetes screening (24-28 weeks) coincides with peak anti-insulin hormone effects from the placenta (hPL, progesterone, cortisol, prolactin).