Of 661 twin gestations studied,221(33.4%) women underwent CL measurement. Of these,101 were randomized to 17OHPC; 120 received placebo.The 25th
%ile CL at 16-20 wks was 36 mm(range 16-72 mm). Clinical characteristics such as race,gravidity,parity,pregravid BMI,prior PTB,chorionicity,assisted reproductive technology,gestational age at randomization were similar between groups. Women with CL ≥25th
%ile were older than those with cervix <25th
%ile(31±6.8 vs 29±6.5 years,p=0.046).The risk for PTB <35 wks was increased for women with short cervix after controlling for treatment(RR 1.5,95%CI 1.12, 2.14,see Table
, *one lost to follow up). 17OHPC did not reduce PTB <35 wks among those with short or long cervix.