Preterm birth rates were 4.1 % ( 22/535 low-dose aspirin ) and 5.7 % ( 31/543 placebo ) ( relative risk [ RR ] 0.72 , 95 % confidence interval [ CI ] 0.42-1 .23 ) ; spontaneous preterm birth rates were 1.1 % ( 6/535 low-dose aspirin ) and 2.2 % ( 12/543 placebo ) ( RR 0.51 , 95 % CI 0.19-1 .34 ) ; medically indicated preterm birth rates were 2.6 % ( 14/535 low-dose aspirin ) and 2.9 % ( 16/543 placebo ) ( RR 0.89 , 95 % CI 0.44-1 .80 ) .