Preterm birth is defined as delivery occurring before 37 completed weeks of gestation and affects about one in ten pregnancies globally. A previous preterm delivery is the single most significant risk factor for a subsequent preterm birth. While a history of delivering early raises the probability of recurrence, it does not guarantee it, and most women will still deliver at term. Understanding the scientific data behind this recurrence risk is the first step in managing a subsequent pregnancy.
Understanding the Recurrence Rate
The baseline risk of preterm birth for a first-time mother is approximately 9% to 10%. This risk is significantly elevated for a woman who has already had one preterm birth, with the average risk of recurrence rising to about 22% in the next pregnancy. This represents a two- to threefold increase in risk compared to women who have previously delivered at term.
The risk is not uniform and changes depending on the circumstances of the first delivery. If the first preterm birth occurred very early (before 32 weeks), the risk of recurrence is higher than if the first delivery was closer to term (between 34 and 36 weeks). The recurrence risk also compounds with each subsequent preterm delivery. Even if a woman delivers a subsequent baby at term, the risk of preterm birth in a third pregnancy remains elevated compared to the general population.
Primary Risk Factors Influencing Subsequent Pregnancies
The elevated statistical risk is driven by identifiable medical and historical factors that influence uterine and cervical function. One major predictor is a short cervical length, which is measured during the second trimester using a transvaginal ultrasound. A cervical length of less than 25 millimeters in the mid-trimester is a strong indicator of increased risk for spontaneous preterm birth.
Another significant factor is the interval between pregnancies; a short inter-pregnancy interval (IPI) is independently associated with a higher risk of recurrence. An IPI of less than 18 months between delivery and the conception of the next pregnancy is linked to an increased chance of preterm birth. This suggests the body may need sufficient time to recover and replenish nutritional stores.
A history of surgical procedures involving the cervix or uterus can also contribute to recurrence risk. Prior procedures, such as a loop electrosurgical excision procedure (LEEP) or cone biopsy performed to treat abnormal cervical cells, can potentially weaken the structural integrity of the cervix. The nature of the first preterm birth also matters, as a spontaneous preterm birth is much more likely to recur than a medically indicated preterm birth.
Clinical Strategies for Prevention
Given the substantial recurrence risk, specific, evidence-based medical interventions are employed during subsequent pregnancies to mitigate the chance of another preterm birth. One of the most common and effective preventative treatments is progesterone supplementation. Progesterone, a naturally occurring hormone, is believed to help maintain uterine quiescence and strengthen the cervix.
For women with a history of spontaneous preterm birth, progesterone is typically administered starting in the late first or early second trimester and continued until around 36 weeks of gestation. This treatment can be given as a vaginal gel or suppository. For high-risk women whose mid-trimester transvaginal ultrasound reveals a short cervix (less than 25 mm), vaginal progesterone treatment significantly reduces the rate of spontaneous preterm birth before 34 weeks.
Another intervention is a cervical cerclage, which involves placing a suture, or stitch, around the cervix to provide mechanical support. A cerclage is typically indicated for women with a history of a very early preterm birth or mid-trimester pregnancy loss consistent with cervical insufficiency. It is also used for those found to have a very short cervix on ultrasound. The procedure is usually performed in the second trimester and the stitch is removed near term to allow for delivery.
Enhanced surveillance is also a standard part of care for women with a history of preterm birth. This monitoring involves more frequent prenatal appointments and serial transvaginal ultrasound measurements of the cervical length, usually beginning around 16 weeks of gestation. This close monitoring allows clinicians to identify cervical shortening early and initiate preventative measures, such as progesterone or cerclage, before the onset of labor.