OBJECTIVE: To evaluate mid-trimester amniotic fluid Interleukin-6 (11-6) levels in twin pregnancies, and to determine if elevated levels are associated with subsequent preterm deliveiy. STUDY DESIGN: We utilized our prenatal diagnosis daiabase from 1988-1995 to identify twin pregnancies with known outcomes and who underwent mitt-trimester ( 14-22 weeks') amniocentesis. Amniotic fluid from the sac of each twin was stored al -70° C until II-0 levels were determined using an EI.ISA assay. Fetal abnormalities and anenploidies were excluded. Preterm deliven was defined as 37% weeks'. RESULTS: Ninety-eight sets of twins were available for analysis. Amniotu lluid IL-6 levels from each twin pair were significanth correlated with each other (R = .54. p < .001 ), Mean IL-6 levels from same sex twim were lower than opposite sex twins (fiOl ±651 vs 1297 ±1949. p = .1). The IL-6 levels in same sex twins had a correlation coefficient of R = .67. p < .001. The cot relation coefficient tor different sex twins was R -.42, p = .04. Neither the mean 1I.-6 level of each Iwin pair nor the highest IL-6 level in each pair préditletl subsequent preterm dcliverv (Table). Logistic regression adjusting for paritv. maternal age, smoking, and weight, and history of prior pielerm birth confirmed this result. MEAXIL-ft GA AT DELIVERY N LEVELS (+ SD) P ≥37 weeks 70 1522 ±1251 <37 weeks 28 1044 ±1220 p=.08 >34 weeks 65 1412 ±1370 <34 w-eeks 33 724 ±770 p = .(K)2 ≥30 weeks 82 1202 r 1302 <30 weeks 16 76H - 766 p=.0:i >14 weeks 93 1170 ±1259 <24 weeks 5 l.'WO = 928 p = .7 CONCLUSION: Mid-triniestfi amniotic fluid IL-6 levels from each twin pair were strongly correlated with same sex twins having a higher correlation coefficient than opposite sex twins. Mid-trimester IL-6 levels were not predictive of subsequent preterm birth.