The World Health Organization (WHO) reports that premature birth rates are climbing in nearly all countries with available data. In the United States, 1 in 10 babies were born prematurely in 2020, according to the Centers for Disease Control and Prevention (CDC).
Although there are many potential causes of preterm births, they can carry a host of harms for babies both early and later in life. The WHO notes that prematurity remains the leading cause of death in children under age 5. Children born prematurely may face physical and developmental delays, while mothers are more vulnerable to a subsequent preterm birth, creating a potentially harmful cycle.
With a clear understanding of risk factors and best practices for assessment, clinicians can give patients a better chance at an uncomplicated pregnancy — and offer their tiniest patients a healthy future.
Understanding the Risk Factors for Preterm Birth
The CDC defines preterm birth as occurring before 37 weeks gestation. The American College of Obstetricians and Gynecologists (ACOG) further classifies prematurity as early preterm (before 34 weeks gestation) and late preterm (between 34 weeks and 36 weeks and six days gestation).
ACOG adds that roughly half of preterm births are preceded by spontaneous preterm labor, with about 25 percent occurring after preterm prelabor rupture of membranes.
Preterm birth originates from a complex cluster of risk factors with overlapping influences. These factors include:
- Socioeconomic status
- Medical complications
- Obstetric history
- Race and ethnicity
- Individual behaviors
- Lack of access to prenatal care
Many preterm birth risk factors appear together, such as smoking and obesity. Some of these factors are particularly prevalent among socioeconomically disadvantaged groups, defying easy prevention.
Racial Health Disparities
As with other areas of maternal health, striking racial disparities appear in preterm birth rates. At 14.4 percent, the rate of preterm births among non-Hispanic Black women is about 50 percent higher than among non-Hispanic white women, according to CDC figures from 2020. Incidence rates are also high for other women of color:
- Non-Hispanic American Indian or Alaska Native women: 10.3%
- Hispanic women: 9.8%
- Non-Hispanic Asian women: 7.29%
Although these same groups experience higher rates of poverty, reports the Kaiser Family Foundation, economic disadvantage cannot solely explain rising preterm birth rates among Black Americans and other people of color. ACOG adds that chronic stress related to long-term exposure to racism may explain the marked difference in preterm birth rates among Black women and those from other racial and ethnic backgrounds.
Lifestyle Factors and Health History
OB/GYNs should assess and counsel all patients at the prepartum and intrapartum stages to identify those who might be at an elevated risk for preterm birth. Early engagement should touch on at-risk behaviors, such as smoking, to encourage positive changes before pregnancy.
Open a dialogue with your patient about modifiable risk factors, such as low maternal pre-pregnancy weight and spacing of fewer than 18 months between pregnancies. With the growing acceptance of recreational and medical marijuana, cannabis usage during pregnancy is the subject of ongoing inquiry, but most reports thus far have found no link between cannabis use and preterm birth.
The nature of your patient's pregnancy may also affect its outcome: Multiples carry a higher risk of preterm delivery than singletons. A previous preterm birth, particularly of twins, increases the risk that the next pregnancy will be preterm. This holds true even for singleton pregnancies.
For those pursuing fertility treatment, a thoughtful conversation about possible pregnancy risks with in vitro fertilization treatment can help to prepare for potential pregnancy complications, including preterm birth.
Other clinical factors and behaviors associated with preterm delivery include:
- Vaginal bleeding
- Urinary tract infection
- Bacterial vaginosis
- Drug and alcohol use
A small pool of studies suggests that women with adenomyosis may experience higher rates of preterm rupture of membranes and preterm birth. Women with congenital uterine anomalies, also called Müllerian duct anomalies, may also be vulnerable to premature delivery.
Your patient's surgical history should also be examined. A meta-analysis of 21 studies and nearly 2 million participants, published in Human Reproduction, reveals an association between a previous dilation and curettage and premature birth, with a slightly greater risk after multiple procedures.
The Gold Standard for Risk Assessment
When assessing risk, ACOG's guidance on spontaneous preterm birth affirms that endovaginal assessment of cervical length provides accurate results. The method, performed in the second trimester, isn't affected by maternal body habitus, fetal shadowing or cervical position, which makes it superior to transabdominal imaging.
The association between a short cervix and preterm birth has been established in diverse populations. ACOG defines a short cervix as 25 mm, usually measured at 16 to 24 weeks gestation.
Cervical shortening earlier in the second trimester is considered a more accurate predictor of preterm birth than shortening beyond 24 weeks gestation. The presence of debris in the intraamniotic fluid has also been linked to preterm birth in women with a short cervix. Other findings, however, including funneling of the os and changes in cervical length over a series of measurements, don't appear to add to the predictive value of cervical length.
When seeking additional warning signs, ACOG states that the presence of fetal fibronectin in cervicovaginal secretions remains a risk factor for preterm birth. Even so, practice guidance advises clinicians to refrain from relying on it as a primary means to identify risk in asymptomatic pregnant patients.
Can Preterm Birth Be Prevented?
Ideally, by identifying at-risk patients, clinicians may recommend steps to prevent preterm birth. Still, a 2019 Cochrane Systematic Review found that identifying symptoms of preterm labor and using that knowledge to manage patients only helped prolong pregnancies by about four days. Noting a shortage of high-quality research in this area, the authors call for further investigations exploring cervical length and prevention of spontaneous preterm birth. Additionally, they encourage inquiries in specific populations, such as among women with and without symptoms of preterm labor and those carrying singletons or multiples.
A 2022 systematic review in the Journal of Medical Internet Research references early investigations into machine learning algorithms as a means to identify the patients most vulnerable to preterm delivery. Similarly, a small study in Ultrasound in Obstetrics and Gynecology suggests artificial intelligence (AI) might help predict preterm birth risk in patients with a short cervix when coupled with ultrasound imaging.
At present, most AI-related inquiries remain in the proof-of-concept phase. Still, clinical applications of AI are expected in the coming years, potentially benefiting shared decision-making and informing patient care in managing and preventing preterm births.
