111 5 MATERNAL CHARACTERISTICS AS INDICATIONS FOR ROUTINE IOL | 3 GOODARZI et al. 2.3 | Determinants The following determinants were examined: maternal ethnicity, age, parity, SES, and gestational age. These determinants have been shown in previous studies to be associated with TM. Maternal age was categorized in 5 years intervals ranging from “younger than 25 years” up to “40 years and older.” Parity was defined in Perined as birth after the gestational age of 16 weeks. Parity was categorized as “P0” (no history of birth), “P1” (history of one birth), “P2” (history of two births), and “P3+” (history of three or more births). Ethnicity was assigned by the woman's care provider, usually based on appearance, name and information provided, and registered according to the following categories: Dutch, North African, African other, Asian other, Latin American, Turkish, and Hindustani. We combined these categories and defined ethnicity as “Dutch” and “non-Dutch.” SES is a score provided by The Netherlands Institute for Social Research, based on the average income, educational level and type of employment in a residential postal code area. We categorized SES as “low,” “medium,” and “high,” using the 25th and 75th percentile cutoff points. Cutoff points were calculated with the SES score and the number of inhabitants per residential postal code area, based on the cutoff points in the general Dutch population provided by the Dutch national statistical office, Statistic Netherlands. Gestational age was categorized in weeks, starting from “pregnant at 37 + 0 weeks” up to “pregnant at 42 + 0 weeks.” The following primary outcomes were examined: TM, FM, and NM.17,18 For the analyses of mortality, we applied an at-risk approach, dividing the number of deaths in a specific week by the number of women still pregnant at the onset of the same week.19 For the analyses of NM, we excluded women with a fetal loss.17 TM was defined as the sum of FM and NM. FM was defined as death occurring before birth. For example, to calculate the FM in week 39, we divided the number of stillbirths at 39 + 0 to 39 + 6 weeks by the number of women still pregnant at 39 + 0 weeks. NM was defined as death up to 28 days after birth among neonates born alive. For example, to calculate NM in week 41, we divided the number of neonatal deaths at 41 + 0 to 41 + 6 weeks by the number of live births among women still pregnant at 41 + 0 weeks. 2.4 | Statistical analyses We used descriptive statistics to report the time trend in single determinants and the rate of TM, FM, and NM in the period 2000-2018. We used descriptive and logistic regression analyses to examine the association between single and multiple determinants and TM in the period 2012-2018. Univariable logistic regression analyses were conducted to calculate crude odds ratio (OR) and 95% confidence intervals (CI). Multivariable logistic regression analyses were conducted to determine ORs adjusted for the other maternal characteristics as potential confounders (aOR). We used seven models. For model 1, we included all women in the study. For models 2-7, results were stratified by gestational week starting from women still pregnant at 37 + 0 weeks up to women still pregnant at 42 + 0 weeks to calculate TM in a specific week among women still pregnant at the start of the week. Collinearity was tested by entering the determinants in the multivariable logistic regression models using a manual stepwise forward–backward method. Inter-categorical analysis was conducted to study the association between multiple maternal characteristics and PM.20,21 The maternal characteristics were entered as interaction terms in the multivariable analysis using a manual stepwise forward–backward method. P-values of <0.05 were considered significant. For some of the analyses, we combined categories because of small sample sizes. For the time trend analyses, we combined the parity categories “P2” and “P3+” and used the categories “younger than 25 years,” “25- 34 years,” and “35 years and older” for age. We did not include model 7– “pregnant at 42 + 0 weeks” –in the regression analyses because of low numbers. The inter- categorical analyses were only conducted for model 1, for F IGURE 1 The trend in fetal, neonatal and total mortality (%) among women pregnant at 37+0 weeks specified by year for the years 2000-2018