966 Seasonality and the Causes of Spontaneous Preterm Birth

The objective of this study was to investigate the relationship between the four seasons in the northern hemisphere and spontaneous preterm birth (SPB). The retrospective study was performed between January 2009 and December 2021. The data of all electronic medical records were retrospectively reviewed in Konkuk University Medical Center, Seoul, Korea. Average temperature and humidity data were obtained from Korea Meteorological Administrations. We excluded the data from women with multi-fetal gestations, pre-eclampsia, gestational hypertension, oligohydramnios, polyhydramnios, GDM, any fetal anomalies, and medico-surgical illnesses that can cause preterm birth. The four seasons are defined as follows: spring (March, April, and May), summer (June, July, and August), fall (September, October, and November), and winter (December, January, and February). The SPB was defined as a fetus born before 36 weeks and 6 days of gestation. SPBs were grouped by the causes of preterm birth: incompetent internal os of the cervix (IIOC), spontaneous preterm labor (PTL), and preterm premature rupture of membranes (PPROM). The SPB rate in relation to IIOC was highest in the summer and lowest in the fall. SPB incidence due to PTL was higher in the fall and winter. SPB incidence due to preterm premature rupture of membranes was highest in spring and summer. The rate of SPB, by considering the etiologies, had different patterns based on the season. Therefore further studies related to seasonal patterns should consider the etiologies of SPB. The unique seasonal characteristics of the region are closed related to the pregnancy outcome of pregnant women, and further studies are required to help improve pregnancy prognosis.