Covariates
Several sociodemographic characteristics and factors related to pregnancy, labour and health care were collected and studied as risk factors for a low Apgar score. Sociodemographic characteristics included maternal age (<25 years, 25-34 years, and ≥35 years), ethnicity (Western vs. non-Western), urbanisation (very urban with >2500 addresses per square kilometre (km2), intermediate with 500 – 2500 households/km2 and very rural with <500 households/km2), and neighbourhood socioeconomic status (SES; in quintiles (Q1 very low/deprived to Q5 very high)).(18) Variables related to pregnancy were: parity (nulliparous P0, multiparous P1-P2 and multiparous P3+), hypertensive disorder including pre-eclampsia, maternal diabetes including gestational diabetes, medical history with caesarean section, and severe vaginal bleeding during gestation.
Variables studied related to labour were: obstetric interventions (induction of labour, instrumental vaginal delivery, planned (primary) and emergency (secondary) caesarean section, and use of epidural analgesia), presentation at birth (cephalic vs. non-cephalic), prelabour rupture of membranes (>24 hours before birth), prolonged second stage of labour (>120 minutes for nulliparous women and >60 minutes for multiparous women), and meconium-stained amniotic fluid. Characteristics at birth were: year of birth, fetal sex, gestational age in weeks, and small (SGA) or large for gestational age (LGA). SGA was defined as <10th percentile and LGA as >90th percentile according to the Dutch reference curves.(19) As health care related factors we studied: time of birth (daytime 8-17 hours, evening 18-24 hours or night 24-8 hours), day of birth on a week or weekend day and level of care (primary vs. secondary at onset of birth and at delivery). Level of care was studied because the Dutch maternity care model is based on risk selection. Antepartum-judged low-risk pregnancies are under surveillance of primary care, whereas high-risk pregnancies are under secondary care. Primary care is provided by midwives, and on a small scale general practitioners, and includes home births, and births in a birth centre or in a hospital under the care of a midwife. Secondary obstetric care is provided by an obstetrician in a general hospital or tertiary academic centre.(20)