Preterm birth rates were @ % ( @/@ low-dose aspirin ) and @ % ( @/@ placebo ) ( relative risk -LSB- RR -RSB- @ , @ % confidence interval -LSB- CI -RSB- @-@ @ ) ; spontaneous preterm birth rates were @ % ( @/@ low-dose aspirin ) and @ % ( @/@ placebo ) ( RR @ , @ % CI @-@ @ ) ; medically indicated preterm birth rates were @ % ( @/@ low-dose aspirin ) and @ % ( @/@ placebo ) ( RR @ , @ % CI @-@ @ ) .