IJE Advance Access originally published online on July 31, 2006
International Journal of Epidemiology 2006 35(5):1169-1177; doi:10.1093/ije/dyl154
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Maternal and biochemical predictors of spontaneous preterm birth among nulliparous women: a systematic analysis in relation to the degree of prematurity
1 Department of Obstetrics and Gynaecology, Cambridge University, Cambridge, CB2 2QQ, UK
2 Medical Research Council Biostatistics Unit, Institute of Public Health, Robinson Way, Cambridge CB2 2SR, UK
3 Department of Public Health, Greater Glasgow NHS Board, Glasgow, G3 8YU, UK
4 Institute of Medical Genetics, Yorkhill NHS Trust, Glasgow, G3 8SJ, UK
5 Information and Statistics Division, Common Services Agency, Edinburgh, EH5 3SE, UK
* Corresponding author. Professor of Obstetrics and Gynaecology, Cambridge University, Rosie Maternity Hospital, Cambridge, CB2 2SW, UK. E-mail: gcss2{at}cam.ac.uk
Background Nulliparous women are at increased risk of spontaneous preterm birth. Other maternal and biochemical risk factors have also been described. However, it is unclear whether these associations are strong enough to offer clinically useful prediction. It is also unclear whether the predictive power of these factors varies in relation to the degree of prematurity.
Methods The risk of spontaneous preterm birth associated with maternal characteristics and second trimester serum screening data was analysed in a dataset of 84 391 first births in Scotland between 1992 and 2001 using Cox and logistic regression. Variation in the relative risk of preterm birth over the period 24–36 weeks was assessed using a test of the proportional hazards assumption.
Results The risk of spontaneous preterm birth was positively associated with maternal serum levels of alpha-fetoprotein, socioeconomic deprivation, number of previous therapeutic abortions, smoking, and being unmarried and was negatively associated with height and body mass index. The risk of preterm birth at 24–28 weeks, but not later gestations, was increased in association with maternal levels of human chorionic gonadotrophin >95th percentile, maternal age <20, and two or more previous miscarriages. The area under the receiver operating characterise curve (95% CI) for models based on these factors was 0.67 (0.63–0.71) for 24–28 weeks, 0.65 (0.62–0.68) for 29–32 weeks, and 0.62 (0.61–0.63) for 33–36 weeks.
Conclusions Time to event analytic methods can identify factors that are differentially associated with spontaneous preterm birth according to the degree of prematurity. However, models based on maternal and biochemical data perform poorly as a screening test for any degree of spontaneous preterm birth.
Keywords alpha-Fetoproteins, chorionic gonadotrophin, labour, premature, proportional hazards models, risk
Accepted 15 June 2006
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