Human placental lactogen (hPL), also called human chorionic somatomammotropin (hCS) or human chorionic somatotropin, is a polypeptide placental hormone, the human form of placental lactogen (chorionic somatomammotropin). Its structure and function are similar to those of human growth hormone. It modifies the metabolic state of the mother during pregnancy to facilitate energy supply to the fetus. hPL has anti-insulin properties. hPL is a hormone secreted by the syncytiotrophoblast during pregnancy. Like human growth hormone, hPL is encoded by genes on chromosome 17q22-24. It was identified in 1963.^[2]

hPL molecular mass is 22 125 Da and contains single chain consisting of 191 amino acid residues that are linked by two disulfide bonds and the structure contains 8 helices. A crystal structure of hPL was determined by X-ray diffraction to a resolution of 2.0 Å.^[1]

hPL is present only during pregnancy, with maternal serum levels rising in relation to the growth of the fetus and placenta. Maximum levels are reached near term, typically to 5–7 mg/L.^[3] Higher levels are noted in patients with multiple gestation. Little hPL enters the fetal circulation. Its biological half-life is 15 minutes. Some women with higher BMI show lower levels of placental lactogen, but whether prenatal health behaviors influence hPL levels or if hPL influences infant birth weight is uncertain.^[4]

hPL affects the metabolic system of the maternal organism in the following manners:

• In a bioassay, hPL mimics the action of prolactin, yet it is unclear whether hPL has any role in human lactation.
• Metabolic:
  • ↓ maternal insulin sensitivity (insulin resistance), leading to an increase in maternal blood glucose levels.
  • ↓ maternal glucose utilization, which helps ensure adequate fetal nutrition (the mother responds by increasing beta cells). Chronic hypoglycemia leads to a rise in hPL.
  • ↑ lipolysis with the release of free fatty acids. With fasting and release of hPL, free fatty acids become available for the mother as free fatty acids do not cross the placenta, so that relatively more glucose can be utilized by the fetus. With sustained fasting, maternal ketones formed from free fatty acids can cross the placenta and be used by the fetus.

These functions help support fetal nutrition even in the case of maternal malnutrition.

hPL is a potent agonist of the prolactin receptor and a weak agonist of the growth hormone receptor.^[5]

While hPL has been used as an indicator of fetal well-being and growth, other fetal testing methods have been found to be more reliable.^{[citation needed]} Also, normal pregnancies have been reported with undetectable maternal levels of hPL.^{[citation needed]}

• Placental lactogen in other species
• Somatotropin family