Iatrogenic and spontaneous preterm birth in England: a population-based cohort study

Objective: To describe the rates of and risk factors associated with iatrogenic and spontaneous preterm birth and the variation in rates between hospitals. Design: Cohort study using electronic health records. Setting: English National Health Service. Population: Singleton births between 1st April 2015 and 31st March 2017. Methods: Multivariable Poisson regression models were used to estimate adjusted risk ratios (adjRR) to measure association with maternal demographic and clinical risk factors. Main outcome measures: Preterm births (<37 weeks gestation) were defined as iatrogenic or spontaneous according to mode of onset of labour. Results: 6.1% of births were preterm and of these, 52.8% were iatrogenic. The proportion of preterm births that were iatrogenic increased after 32 weeks. Both sub-groups are associated with previous preterm birth, extremes of maternal age, socio-economic deprivation and smoking. Iatrogenic preterm birth was associated with higher BMI (adjRR BMI >40 1.59 (1.50, 1.69)), and previous caesarean (adjRR 1.88 (1.83, 1.95)). Spontane-ous preterm birth was less common in women with a higher BMI (adjRR BMI>40 0.77 (0.70, 0.84)) and in women with a previous caesarean (adjRR 0.87 (0.83, 0.90)). More variation be-tween NHS hospital trusts was observed in rates of iatrogenic, compared to spontaneous, pre-term births.

Conclusions: Just over half of all preterm births resulted from iatrogenic intervention. Iatro-genic births have overlapping but different patterns of maternal demographic and clinical risk factors to spontaneous preterm births. Iatrogenic and spontaneous sub-groups should therefore be measured and monitored separately, as well as in aggregate, to facilitate different preven-tion strategies. This is feasible using routinely acquired hospital data.