- Objective: Pentraxin 3 (PTX3) is a soluble pattern recognition receptor (PRR) that has an important role in immunoregulation and vascular integrity. The aim of this study was to determine if PTX3 is present in amniotic fluid (AF) and whether its concentration changes with gestational age (GA), in the presence of preterm or term labor, and in cases of intra-amniotic infection/inflammation (IAI) associated with spontaneous preterm labor (PTL) or preterm prelabor rupture of membranes (PROM). Study design: This cross-sectional study included the following groups: 1) mid-trimester (n=45) ; 2) uncomplicated pregnancies at term with (n=48) and without (n=40) spontaneous labor; 3) women with PTL and intact membranes who: a) delivered at term (n = 44); b) delivered preterm without IAI (n=40); or c) delivered preterm with IAI (n=62); 4) women with preterm PROM with (n=63) and without (n = 36) IAI. PTX3 concentration in AF was determined by ELISA. Non-parametric statistics were used for analyses. Results: 1) Among women with PTL and intact membranes, the median AF PTX3 concentration was significantly higher in women with IAI than in those without IAI (7.95 ng/mL vs. 0.38 ng/mL; P<0.001) and than in those who delivered at term (0.55 ng/mL; P<0.001); 2) women with preterm PROM and IAI had a higher median AF PTX3 concentration than those without IAI (9.12 ng/mL vs. 0.76 ng/mL; P<0.001); 3) the median AF PTX3 concentration did not change with GA (mid-trimester: 0.79 ng/mL vs. term not in labor: 0.58 ng/mL; P=0.09); and 4) labor at term was not associated with a significant change of AF PTX 3 concentration (in labor: 0.54 ng/mL vs. not in labor: 0.58 ng/mL, P=0.9). Conclusions: PTX3 is a physiologic constituent of the AF, and its median concentration is elevated in the presence of IAI, suggesting that PTX3 may play a role in the innate immune response against IAI.