Six lysosomal hydrolases were studied in tissues of human placenta, amnion,
decidua, and myometrium obtained in the 2nd and 3rd trimester. No significant increase in
the activity of any lysosomal hydrolase was found in the 3rd-trimester values compared with
those obtained in the 2nd trimester. Thus, the increased activity of lysosomal hydrolases in
maternal serum at term, previously described in several studies, seems not to be paralleled by
a corresponding increase in the activity of these enzymes in the tissues of the pregnant uterus.
We speculate that the increase in maternal serum towards term may reflect a macrophage
activation initiated by elevated estrogen concentrations in the 3rd trimester.