Human placental lactogen

Human placental lactogen (HPL), also called human chorionic somatomammotropin, is a polypeptide placental hormone. Its structure is related to that of human growth hormone. It modifies the metabolic state of the mother during pregnancy to facilitate the energy supply of the fetus.

Structure

HPL consists of 190 amino acids that are linked by two disulfite bonds and is secreted by the syncytiotrophoblast during pregnancy. Its molecular weight is 22,125. Like human growth hormone HPL is encoded by genes on chromosome 17q22-24. Its biologic half-life is 15 minutes.

Levels

HPL is only present 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/ml. Higher levels are noted in patients with multiple gestation. Little HPL enters the fetal circulation.

Function

In a bioassay HPL mimics the action of prolactin, yet it is unclear if HPL has any role in human lactation.

HPL affects the metabolic system of the maternal organism. HPL increases production of insulin and IGF-1 and increases insulin resistance and carbohydrate intolerance. Chronic hypoglycemia leads to a rise in HPL. HPL induces lipolysis with the release of free fatty acids, increase in insulin secretion and insulin resistance. With fasting and release of HPL, free fatty acids become available for the maternal organism as fuel, so that relatively more glucose can be utilized by the fetus. Also, ketones formed from free fatty acids can cross the placenta and be used by the fetus. These events support energy supply to the fetus in states of starvation.

Clinical measurement of HPL

While HPL has been used as an indicator of fetal well-being and growth, other fetal monitoring methods have been found to be more reliable. Also, normal pregnancies have been reported with undetectable maternal levels of HPL.

References

Speroff L, Glass RH, Kase NG. Clinical Gynecologic Endocrinology and Infertility. Sixth edition. Lippincott Williams & Wilkins, Baltimore, MD 1999. ISNB 0-683-30379-1.

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