| Time of folic acid supplementation | sPTD | Unadjusted | Adjusted 1 | ||||
---|---|---|---|---|---|---|---|---|
 |  | n | HR | (CI) | p | HR | (CI) | p |
All | >8 w preconception | 478 | 1.17 | (1.06; 1.30) | 0.003 | 1.17 | (1.04; 1.32) | 0.01 |
 | 0-8 w preconception | 648 | 1.01 | (0.92; 1.12) | 0.80 | 1.01 | (0.90; 1.13) | 0.84 |
 | 1st trimester | 1326 | 0.94 | (0.84; 1.05) | 0.26 | 0.89 | (0.79; 1.00) | 0.05 |
 | 2nd trimester | 869 | 1.10 | (1.00; 1.21) | 0.04 | 1.08 | (0.98; 1.19) | 0.11 |
Early | >8 w preconception | 108 | 1.49 | (1.19; 1.88) | 0.001 | 1.51 | (1.16; 1.97) | 0.002 |
 | 0-8 w preconception | 133 | 1.15 | (0.92; 1.43) | 0.23 | 1.03 | (0.80; 1.34) | 0.81 |
 | 1st trimester | 262 | 1.11 | (0.85; 1.44) | 0.45 | 0.99 | (0.74; 1.32) | 0.95 |
 | 2nd trimester | 173 | 1.21 | (0.97; 1.49) | 0.09 | 1.12 | (0.89; 1.39) | 0.34 |
Late | >8 w preconception | 370 | 1.10 | (0.98; 1.24) | 0.10 | 1.10 | (0.97; 1.26) | 0.15 |
 | 0-8 w preconception | 515 | 0.98 | (0.88; 1.09) | 0.69 | 1.00 | (0.88; 1.13) | 0.94 |
 | 1st trimester | 1064 | 0.91 | (0.81; 1.03) | 0.13 | 0.87 | (0.76; 1.00) | 0.04 |
 | 2nd trimester | 696 | 1.08 | (0.98; 1.20) | 0.13 | 1.08 | (0.97; 1.20) | 0.17 |