Risk factors associated with preterm birth according to gestational age at birth

PURPOSE: To identify and quantify risk factors associated with preterm birth, stratified by gestational age at birth. METHODS: Three case-control analyses were done. Controls were pregnancies of > or =37 weeks of gestational age at birth. Cases were defined as: <28, 28-32, 33- <37 weeks of gestational age at birth respectively in the three case-control analyses. Women were categorized according to whether they carried single or multiple infants. RESULTS: Obstetrical conditions (placenta previa, placental abruption), and maternal hypertension were significantly associated with preterm delivery in all case-control analyses (adjusted OR between 1.34-19.56, p < 0.05). Leading risk factors for preterm delivery in singleton pregnancies were placental abruption and placenta previa (adjusted ORs 4.85 and 4.13, p < 0.05). For multiple pregnancies they were polyhydramnios and maternal hypertension (adjusted ORs 4.39 and 2.45, p < 0.05). CONCLUSIONS: Obstetrical conditions during the pregnancy and maternal hypertension are important risk factors for preterm birth.