Study design and population
We conducted a nationwide cohort study with data from the national Perinatal Registry of the Netherlands (PERINED). This registry contains data on pregnancy, delivery and neonatal outcome and covers 97% of all deliveries after 16 weeks of gestation in the Netherlands.10 The PERINED database is obtained by linkage of three registries: the midwifery registry (LVR-1), obstetrics registry (LVR-2) and the neonatology registry (LNR). Permission for usage of the PERINED data with the purpose of this study was obtained on July, 13 2021 (approval number 21.13).
We extracted the data of registered pregnancies that ended in birth > 16 weeks of gestation between 2010 and 2014 (index pregnancies). We excluded pregnancies from primiparous or multiparous women, multiple pregnancies, pregnancies with birth > 28 weeks of gestation or pregnancies with an induced start of labor. Subsequently, the included pregnancies were linked with a 5 year cohort in primiparous women. This was done separately for each year with a subsequent 5-year cohort in the years 2010-2019. The 3 deterministic linkage keys were date of birth of the mother, data of previous birth/ birth date of the child and 4-digid zip code. Linkage was possible if two out of three variables were available of which maternal birth day was one of them. After linkage of prior birth and subsequent birth, prior pregnancies that were complicated with antenatal diagnosed IUFD and/or congenital anomalies were excluded from the sPTB index group in the analysis.