Human Placental Lactogen (HPL)

HPL* is another of the protein hormones formed by the placenta. It is similar to the pituitary growth hormone.

HPL can be detected in the serum at the 6th week of pregnancy, and it's levels continue to rise with growth of the placenta. In pregnancies with smaller placentas, there is less HPL detected. The highest concentration in the blood occurs in late pregnancy.

The functions of HPL are to:

• help prepare the breasts for lactation. It stimulates breast growth and development and stimulates the secretion of colostrum.
• to decrease the mother's use of glucose, so that it can be used by the fetus for growth and development.

The latter function occurs several ways:

• HPL promotes breakdown of maternal fats, which results in a higher level of maternal fatty acids in the plasma. The mother uses the fatty acids for energy, "saving" the glucose for the fetus.
  • HPL is often thought of as an "insulin antagonist." As the level of HPL increases, it causes an increasing resistance of the mother's tissue to insulin. Since insulin is needed to transfer glucose into the mother's cells, her "resistance" causes the level of glucose to rise in her blood. HPL reaches it's peak at about the 23rd week of pregnancy, when insulin production may not be able to keep up with the high demands of the pregnancy. Obviously, the result is a tendency toward maternal diabetes in pregnancy.
• HPL also affects protein metabolism, so more amino acids are available for the fetus. It does this by decreasing the mother's use of protein for energy and increasing synthesis of protein. This creates a source of amino acids for transport to the fetus, resulting in increased levels of protein available for fetal growth and development.
  • Since the mother's tissues are resistant to insulin, the pancreas is called upon to secrete additional insulin. This additional insulin promotes protein synthesis, which again produces additional amino acids for the fetus.

There will be more information on insulin, the pancreas, and diabetes later in the course.

*also known as human chorionic somatotropin (HCS)