Forest plots of odds ratios for preterm birth in Scotland by epoch.

<p>(A) Unadjusted odds ratio for a one-unit increase in number of previous abortions (coded as 0, 1, 2, and 3 or more) in relation to risk of preterm first birth among 732,719 women for births from 1980 to 2008. (B) As in (A), but odds ratio adjusted for maternal characteristics (deprivation category, previous miscarriage, maternal age, height, and marital status). (C) Adjusted odds ratio for a one-unit increase in number of previous abortions in relation to risk of preterm first birth among 414,373 women for births from 1992 to 2008. Odds ratios adjusted for maternal characteristics as in (B), but also for smoking. (D) Adjusted odds ratio for a 10-cm decrease in maternal height in relation to the risk of preterm first birth among 732,719 women for births from 1980 to 2008. Odds ratios adjusted for deprivation category, maternal age, marital status, previous abortion, and previous miscarriage. The interaction <i>p</i>-value is for a Wald test of the null hypothesis that the odds ratios did not significantly differ across the period 1980 to 2008. Year is treated as a continuous variable in all the statistical tests of interaction.