raised alkaline phosphatase in pregnancy

Last reviewed 01/2018

Alkaline phosphatase (AP) is known to be produced by the liver, bones, small intestine, and kidneys, and different AP isoforms are also expressed by the placenta during pregnancy

  • average amount of AP in one human term placenta amounts to 40 mg

  • placental isoforms are known as heat stable AP, because they are heat resistant at 60°C, a property that is the main criterion for distinguishing them from the other isoenzymes

  • in early pregnancy, the tissue non-specific AP isoenzyme is mainly expressed in the placenta, and reaches a peak value around 10 weeks of pregnancy

  • at the end of the second trimester, most of the AP activity comprises term placental AP isoenzymes (90% of which are the P1 type, 10% the P2 type) produced by the syncytiotrophoblasts, and these isoenzymes appear in maternal serum between the 15th and 26th weeks of pregnancy
    • their plasma concentrations increase exponentially during gestation - they are present at concentrations three times greater than those seen in non-pregnant women - and have a long half life (seven days) postpartum

    • extremely high AP concentrations should arouse a suspicion of bone, hepatic, endocrine, and renal diseases, malignancy, and drug treatment, but can also be associated with heavy smoking and pregnancy (1)

    • in a pregnant patient, elevation of ALP may be related to HELLP syndrome and intrahepatic cholestasis (2)

    • AP electrophoresis can be useful to distinguish placental from hepatic or bone isozymes

    • usually, AP production or diffusion in maternal serum is not major and total serum ALP level remains normal (2)
      • some cases of unusual elevation of extremely high levels placental AP have been described (3,4)

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Related pages:

asymptomatic raised alkaline phosphatase