The sample size was calculated based on the
proportion of preterm deliveries between the unmatched
case-control.28-30 According to a 95% confidence interval
(CI), 90% power, ratio of control: cases 4.0, odds ratio 2,
sample size would be 600 for case and 2400 for control
with a 3000 in total. In our study, Six cases with
incomplete information were excluded. Additional of six
were add in the control group. Thus, 594 were cases and
2406 unmatched controls.