What does the preterm birth data look like for your circumstances?

A large population-based study by Zhang X et al. (2019, PLoS ONE, N=10,333,501) found a relative risk of 1.10 to 1.12 for spontaneous preterm birth in subsequent pregnancies following a caesarean. This is a modest increase. At a baseline rate of around 10%, a 1.11 multiplier moves the population estimate to approximately 11.1%. For the majority of people with a previous caesarean, this increment is small and full-term delivery remains the most likely outcome.

Cervical length is measured by transvaginal ultrasound, typically at the 18 to 24 week anatomy scan. A short cervix (below 25 mm in a singleton pregnancy) is associated with increased preterm birth risk. For people with a prior preterm birth, cervical length monitoring is recommended by RCOG and ACOG guidelines because it allows timely intervention. Where cervical length is found to be short, vaginal progesterone has been shown in randomised trials to reduce preterm birth risk in appropriate candidates. Cervical cerclage (a stitch to support the cervix) is recommended in specific clinical scenarios. These are well-evidenced, manageable interventions. Monitoring is not about finding problems: it is about finding the people who benefit most from targeted support.

Early term birth (37 to 38 weeks) is technically full term by the standard definition of 37 or more weeks, but it sits at the lower end of the term spectrum. Some research suggests that a prior birth in this range may be a mild marker of a tendency toward earlier delivery, and it is reflected as a modest modifier in this calculator. In most cases, a single birth at 37 to 38 weeks is reassuring: it was still a term birth, and the majority of subsequent pregnancies will also deliver at or near term.

The most consistently supported risk factors in peer-reviewed literature are prior preterm birth, multiple gestation (twins, triplets), a short cervix detected on ultrasound, uterine or cervical abnormalities, urinary tract infections and bacterial vaginosis during pregnancy, and smoking. The March of Dimes identifies prior preterm birth as the single strongest individual predictor, approximately doubling population-level risk for a subsequent pregnancy. Multiple gestation carries a preterm birth rate of approximately 60%, compared to the 10.4% rate in singleton pregnancies. Smoking is associated with a relative risk of approximately 1.3 across multiple studies.

Yes, and they are substantial. CDC NCHS 2023 data shows that the preterm birth rate for non-Hispanic Black women in the US is 14.6%, compared to 9.4% for non-Hispanic white women. This gap has persisted for decades and is not explained by individual risk factors alone. Research published in the American Journal of Obstetrics and Gynecology (Alhusen et al., 2016) and subsequent work has linked the disparity to chronic stress from racism, differential access to quality antenatal care, neighbourhood-level poverty, and biological pathways including elevated inflammatory markers. The disparity is a public health priority and the subject of active policy and research attention.

The relationship between psychological stress and preterm birth is real but complex. A 2019 systematic review in BMC Pregnancy and Childbirth found that chronic stress, particularly stress related to racism and socioeconomic disadvantage, was associated with modestly elevated preterm birth risk. Acute life events showed a weaker and less consistent signal. The proposed biological mechanism involves cortisol-mediated activation of the hypothalamic-pituitary-adrenal axis and inflammatory cytokines that can trigger cervical ripening prematurely. However, the effect size is modest and stress is not a reliable independent predictor at the individual level. Most people experiencing significant stress during pregnancy deliver at term.

Survival rates vary substantially by gestational age. Data from the Eunice Kennedy Shriver NICHD Neonatal Research Network shows that survival at 22 weeks gestation is approximately 10 to 20% with active intervention, rising to around 50 to 55% at 23 weeks, 70 to 80% at 24 weeks, and over 90% at 26 weeks. By 28 weeks, survival exceeds 95% at centres with neonatal intensive care. These figures represent outcomes at specialist centres and may differ at hospitals without high-level NICU facilities. Survival without major morbidity is a separate and lower figure at the earliest gestational ages.

Vaginal progesterone is a well-evidenced intervention for reducing preterm birth risk in two specific populations: people with a short cervix (25 mm or less) detected on transvaginal ultrasound in a singleton pregnancy, and people with a prior spontaneous preterm birth. The landmark PREGNANT trial (Hassan et al., 2011, N=458) showed that vaginal progesterone micronised gel reduced preterm birth before 33 weeks by 45% in women with a short cervix. The ACOG and RCOG both recommend it for these indications. It does not benefit unselected populations with no identified risk factors, and there is no evidence it is helpful as a general preventive measure.

NICU length of stay is closely linked to gestational age at birth. A general clinical rule of thumb is that a preterm infant is expected to be discharged around their original due date, meaning a baby born at 28 weeks (12 weeks early) may spend approximately 10 to 14 weeks in the NICU. Late preterm infants (34 to 36 weeks) typically require 2 to 6 weeks of care, often focused on feeding establishment and temperature regulation rather than respiratory support. These are population averages: individual stays vary based on complications, infection, or surgical needs. Most late preterm infants born at 35 to 36 weeks are discharged within 2 to 3 weeks if uncomplicated.

Long-term outcomes are strongly differentiated by gestational age. Late preterm infants (34 to 36 weeks) have outcomes broadly similar to term infants, with a modestly elevated risk of learning difficulties and attention problems at school age. Very preterm infants (under 32 weeks) have higher rates of neurodevelopmental impairment: a 2017 meta-analysis in JAMA Pediatrics (Van Lieshout et al.) found that 40 to 50% of extremely preterm survivors had some form of neurodevelopmental difficulty, ranging from mild to severe. The majority of late preterm infants, who make up the largest share of all preterm births, enter adult life without significant disability linked to their prematurity.

Clinical definitions are standardised internationally. Preterm birth is delivery before 37 completed weeks of gestation. Within this, late preterm covers 34 to 36 weeks and accounts for approximately 70% of all preterm births. Very preterm is defined as birth before 32 weeks, and extremely preterm as birth before 28 weeks. The overall US preterm birth rate of 10.4% is dominated by late preterm cases, which typically require shorter NICU stays and have better long-term outcomes. Extremely preterm births, at under 28 weeks, are rare at around 0.4% of all US births but account for a disproportionate share of neonatal mortality and long-term morbidity.