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Preterm labor is a major cause of preterm delivery.
The cause of most cases of labor is unknown. Risk factors include low socioeconomic status, nonwhite race, maternal age of 18 years or less or of 40 years or more, and low prepregnancy weight. Women who have experienced one or more spontaneous second-trimester abortion are at increased risk for subsequent deliveries. Maternal smoking, cocaine use, and lack of prenatal care are associated with delivery. It presents as painless cervical dilation and premature loss.