Alkaline phosphatase (ALP) is an enzyme present in tissues like the liver, bones, kidneys, and intestines. A routine blood test, often part of a comprehensive metabolic panel, measures the amount of this enzyme in the bloodstream. Doctors use this measurement as a marker to assess the function of certain organs, particularly the liver and bones.
The Placenta’s Role in ALP Levels
During pregnancy, a significant and expected increase in total ALP levels occurs due to the placenta. This organ produces its own unique form of the enzyme, known as placental alkaline phosphatase (PLAP). This isoenzyme enters the maternal bloodstream around the beginning of the second trimester, and its concentration rises steadily throughout the pregnancy.
The production of PLAP is a sign of a developing and functioning placenta. While its exact functions are still under investigation, researchers believe it is involved in placental growth and the transport of nutrients to the fetus. This natural rise in ALP is a normal physiological change associated with a healthy pregnancy.
Normal ALP Ranges During Pregnancy
Standard reference ranges for ALP do not apply during pregnancy because of the additional contribution from the placenta. As pregnancy progresses, the acceptable upper limit for ALP increases substantially. This rise begins after the first trimester and continues to climb, often peaking in the third trimester just before delivery.
ALP levels during a healthy pregnancy can be two to three times higher than the upper limit for non-pregnant individuals. For instance, if a standard lab range is 47 to 147 IU/L, levels in the third trimester can be normal even if they exceed 400 IU/L. A healthcare provider must interpret these results, considering the specific gestational age and the individual’s baseline levels.
When High ALP May Indicate a Complication
While a steady rise in ALP is normal, an unusually high or sharply increasing level can signal an underlying issue. Extremely elevated ALP may be investigated as a marker for placental insufficiency, a condition where the placenta cannot adequately deliver oxygen and nutrients. This may be associated with intrauterine growth restriction (IUGR).
Certain serious pregnancy-related conditions can also be associated with abnormally high ALP levels. Intrahepatic cholestasis of pregnancy (ICP) is a liver disorder that develops in late pregnancy and can cause a significant spike in ALP. Preeclampsia and HELLP syndrome are other complications that can disrupt liver function and lead to elevated ALP.
A doctor will use the ALP level as one piece of information, not as a standalone diagnostic tool. If levels are concerningly high, further tests are often ordered to determine the enzyme’s source—whether it’s the placental type (PLAP) or from the liver or bone. This helps distinguish a normal increase from a potential health complication.
Postpartum ALP Levels
Following childbirth, alkaline phosphatase levels begin to decline. The delivery of the placenta removes the source of the placental alkaline phosphatase (PLAP) that caused the elevation, allowing total ALP to return toward the individual’s non-pregnant baseline.
This normalization process typically occurs within the first few weeks after delivery. By about six weeks postpartum, ALP levels have usually returned to the standard range. This confirms that the high levels were a temporary and normal part of the gestational period.