Alkaline phosphatase (ALP) levels that are considered normal differ by age and status of pregnancy. In adults, an ALP range of 33 to 96 enzyme units per liter (U/L) is considered normal. For children, this range is under 350 U/L. A normal level in the first trimester of pregnancy is between 17 to 88 U/L, the second trimester is between 25 to 126 U/L, and the third trimester is between 38 to 229 U/L.