Which factor is a well-documented risk for preterm labor?

A history of preterm birth is a well-documented risk factor for subsequent preterm labor because it indicates that a woman has previously experienced the conditions or circumstances that can lead to early delivery. The biological mechanisms that contribute to preterm labor can be multifactorial and may involve factors such as uterine abnormalities, infections, or cervical incompetence, which can persist or reoccur. As a result, women who have had one preterm birth are at a significantly increased risk for having another, with estimates suggesting that the risk increases to around 30-40% for subsequent pregnancies. This risk is recognized in clinical settings, where healthcare providers closely monitor and manage pregnancies in women with a prior history of preterm birth to the best of their abilities.