Is maternal serum heat stable alkaline phosphatase an index of fetal maturity?

A cross sectional estimation of serum heat stable alkaline phosphatase (HSAP) levels in 145 women with uncomplicated pregnancies between 24-41 weeks gestation showed increasing levels with advancing gestation and a steep rise at about 34 weeks gestation from a mean level of 17.3±6.5 μg/ml to a mean level of 36.1±1.5 μg/ml (t = 7.4 p < .001) at about 37 weeks gestation. Ninety-three point seven percent (93.7%) and 24.4% of women between 31-35 weeks gestation and 27-34 weeks gestation respectively had levels equal to or exceeding 20 μg/ml. It is postulated that serum HSAP represent stages of fetal maturity rather than gestational age per se and that there is a possible relationship between serum HSAP and amniotic fluid lecithin viewed against a background of a similar rise of amniotic fluid lecithin levels at about 33 weeks gestation and the common role of steroid receptor sites in the formation of both HSAP and Lecithin.