In logistic regression analysis controlling for FFN , cervical length , prior preterm birth , and treatment group , positive FFN was significantly associated with birth < 30 and < 32 weeks ( odds ratio 55.0 [ 95 % confidence interval 5.2-582 ] , 18.1 [ 3.3-99 ] , respectively , p < 0.001 for both ) but cervical length 25 mm was not ( odds ratio 0.1 [ 0.002-1 .6 ] , 0.6 [ 0.1-4 .3 ] ) .