Objective. The fetus is thought to play a central role in the onset of labor. Pulmonary surfactant protein (SP)-A, secreted by the maturing fetal lung, has been implicated in the mechanisms initiating parturition in mice. The present study was conducted to determine whether amniotic fluid concentrations of SP-A and SP-B change during human parturition.

Study design. Amniotic fluid SP-A and SP-B concentrations were measured with a sensitive and specific ELISA in the following groups of pregnant women: (1) mid-trimester of pregnancy, between 15 and 18 weeks of gestation (n = 29), (2) term pregnancy not in labor (n = 28), and (3) term pregnancy in spontaneous labor (n = 26). Non-parametric statistics were used for analysis.

Results. SP-A was detected in all amniotic fluid samples. SP-B was detected in 24.1% (7/29) of mid-trimester samples and in all samples at term. The median amniotic fluid concentrations of SP-A and SP-B were significantly higher in women at term than in women in the mid-trimester (SP-A term no labor: median 5.6 μg/mL, range 2.2–15.2 μg/mL vs. mid-trimester: median 1.64 μg/mL, range 0.1–4.7 μg/mL, and SP-B term no labor: median 0.54 μg/mL, range 0.17–1.99 μg/mL vs. mid-trimester: median 0 μg/mL, range 0–0.35 μg/mL; both p < 0.001). The median amniotic fluid SP-A concentration in women at term in labor was significantly lower than that in women at term not in labor (term in labor: median 2.7 μg/mL, range 1.2–10.1 μg/mL vs. term no labor: median 5.6 μg/mL, range 2.2–15.2 μg/mL; p < 0.001). There was no significant difference in the median amniotic fluid SP-B concentrations between women in labor and those not in labor (term in labor: median 0.47 μg/mL, range 0.04–1.32 μg/mL vs. term no labor: median 0.54 μg/mL, range 0.17–1.99 μg/mL; p = 0.2).

Conclusion. The amniotic fluid concentration of SP-A decreases in spontaneous human parturition at term.