Preterm birth, defined as delivery before 37 completed weeks of gestation, is a significant global health challenge. A baby born too early faces an elevated risk of both short-term health complications and long-term developmental issues. This article focuses specifically on spontaneous preterm birth, which accounts for the majority of early deliveries and happens without medical intervention.
Understanding the Timing of Delivery
Preterm birth is classified based on how early the delivery occurs, which correlates with the potential health risks for the infant. The World Health Organization categorizes these births into three groups:
- Extremely preterm (before 28 weeks)
- Very preterm (between 28 and less than 32 weeks)
- Moderate to late preterm (between 32 and 37 weeks)
Spontaneous preterm birth (SPTB) is distinct from medically indicated preterm birth. SPTB occurs when labor begins on its own, either through regular uterine contractions causing cervical change or premature rupture of the membranes. In contrast, a medically indicated preterm birth is initiated by a healthcare provider, often through induction or cesarean delivery, due to a health risk to the mother or the fetus.
Common reasons for a medically indicated delivery include conditions like severe preeclampsia, placental abruption, or intrauterine growth restriction. Most preterm births, approximately two-thirds, are spontaneous, but the underlying causes for this process are often complex and not fully understood. The distinction between spontaneous and indicated subtypes is important for research and for determining the most appropriate prevention strategies.
Identifying Factors that Increase Risk
The underlying causes of spontaneous preterm birth are complex, involving multiple pathways such as infection, inflammation, and uterine overdistension. While many women who deliver early have no identifiable risk factors, several established conditions significantly increase the likelihood of SPTB.
The single greatest risk factor is a history of a previous spontaneous preterm birth, which can increase the recurrence risk by 1.5- to 2-fold. The risk is even higher for women who have had multiple previous SPTBs or a delivery at a very early gestational age. A short interval between pregnancies, specifically less than six months, also contributes to an elevated risk.
Factors related to the current pregnancy include carrying multiple fetuses, such as twins or triplets, which is a strong risk factor due to the mechanical stress of uterine overdistension. Structural issues with the reproductive organs, including a short cervix or abnormalities of the uterus, also predispose a woman to SPTB.
Maternal health and lifestyle choices play a role in risk assessment. Certain infections, including genitourinary or periodontal disease, have been linked to an increased chance of early delivery. Lifestyle factors like smoking, being significantly underweight, or being exposed to high levels of stress are also associated with an elevated risk of SPTB.
Clinical Screening and Predictive Tools
Screening for spontaneous preterm birth involves identifying patients at elevated risk before they experience symptoms of labor, allowing for the timely implementation of preventative therapies. The most reliable clinical tool for predicting SPTB in asymptomatic women is the transvaginal ultrasound measurement of cervical length.
The cervix naturally shortens before labor begins. A cervical length of 25 millimeters or less in the mid-trimester is considered a strong predictor of increased SPTB risk, with shorter measurements indicating a greater likelihood of early delivery. This measurement is often performed during the routine anatomy scan, typically between 18 and 24 weeks of gestation.
Another predictive tool involves biochemical markers, particularly the fetal fibronectin (fFN) test. Fetal fibronectin is a protein that acts as “glue” between the fetal membranes and the uterus. A negative fFN test in a symptomatic patient indicates a very low probability of delivery within the next one to two weeks.
A positive fFN test, where the protein is detected in cervicovaginal fluid, suggests a disruption of the interface and an increased risk of SPTB. While fFN is not recommended for routine screening in low-risk women, it is a valuable tool when a patient presents with symptoms of preterm labor. Newer research is exploring the use of protein biomarkers in the blood to identify at-risk women, even in the first trimester of pregnancy.
Current Prevention and Management
For women identified as high-risk through screening or history, specific medical interventions are available to prevent spontaneous preterm birth. Progesterone supplementation is the most common and effective preventative strategy, as this hormone helps maintain uterine quiescence and strengthen the cervix.
Progesterone is typically administered starting in the second trimester and continued until about 34 weeks of gestation. For women with a history of SPTB, it may be given as a weekly intramuscular injection or a daily vaginal preparation. Vaginal progesterone is the preferred treatment for women who have a short cervix but no prior history of early delivery.
A surgical option, known as a cervical cerclage, may be considered for women with a history of SPTB or a very short cervix. This procedure involves placing a stitch around the cervix to provide mechanical support, preventing premature opening and dilation. Cerclage is generally reserved for patients with the most significant risk factors, such as a prior SPTB and a cervical length less than 20 millimeters.
If spontaneous preterm labor cannot be stopped, the focus shifts to optimizing outcomes for the newborn. Tocolytics are medications used to temporarily relax the uterus and delay delivery for up to 48 hours. This brief delay is crucial as it provides time to administer a course of corticosteroids, which accelerate the maturation of the fetal lungs.
Corticosteroids are generally recommended for pregnancies between 24 and 34 weeks of gestation and significantly reduce the risk of neonatal respiratory distress syndrome. Additionally, magnesium sulfate is given to women delivering before 32 weeks for fetal neuroprotection, reducing the risk of cerebral palsy and other severe neurodevelopmental impairments in the preterm infant.