Objectives: To confirm that the bone isoenzyme of alkaline phosphatase (ALP) contributes significantly to the increased ALP in normal pregnancy, and to determine the gestational age at which the increase occurs.
Methods: Cross-sectional determinations of serum total, bone and placental ALP were carried out in 67 normal pregnant women. The results were compared with those of 18 normal age-matched non-pregnant controls using Student's t-test.
Results: At both 31-32 weeks and 38 weeks of gestation, serum activities of bone ALP were increased significantly (P < 0.05 and P < 0.001, respectively) when compared with controls. The percentage contribution of bone ALP to the total was higher at 31-32 weeks and lower at 38 weeks when compared with the contribution from placental ALP.
Conclusion: Knowledge of time of elevation of serum bone ALP activity in normal pregnancy will be helpful in the interpretation of elevated serum total ALP activity during pregnancy.