After restriction to confirmed pregnancies using inverse probability weighting , preterm birth rates were 5.7 % and 9.0 % ( RR 0.63 , 95 % CI 0.37-1 .09 ) and spontaneous preterm birth rates were 1.4 % and 3.2 % ( RR 0.44 , 95 % CI 0.17-1 .18 ) .