For diagnosis of cases, we used the diagnosis of
preterm delivery using American College of Obstetricians
and Gynecologists(ACOG) guideline31 and the
gestational age was based on the last menstrual period
(LMP) or confirm with ultrasonography examination. If
the diagnosis of preterm delivery from both LMP and
ultrasound dating (before 20 weeks gestation) were
available and the two agreed within 14 days. The former
was used to assign gestational age. If the two date
differed by more than 14 days and the latter was used.