Unintended pregnancy is associated with a 26.7% increased risk of preterm birth, according to a large-scale analysis of more than 900,000 pregnancies across 37 low- and middle-income countries.
Analysis of More Than 900,000 Pregnancies Worldwide
Unintended pregnancies, encompassing those categorized as mistimed or unwanted at the time of conception, remain a central challenge in global public health. To better understand their physiological impacts, researchers examined Demographic and Health Surveys gathered between 1997 and 2021 across 37 low- and middle-income nations. The dataset comprised 911,423 pregnancies among 635,139 women aged 15 to 49 years, with 205,655 pregnancies—representing 22.6% of the total—classified as unintended.
Rather than depending strictly on conventional observational models, the investigative team deployed target trial emulation. This statistical method aims to replicate specific characteristics of a randomized controlled trial using observational data. By applying statistical weighting, the researchers balanced key variables between intended and unintended pregnancies, including maternal age, body mass index (BMI), education level, household wealth, marital status, smoking habits, place of residence, reproductive history, and familiarity with family planning methods.
Preterm Birth Risk Rises by 26.7% in Emulation Model
The target trial emulation revealed that unintended pregnancy carried a 26.7% higher relative risk of preterm delivery compared to intended pregnancy. Specifically, preterm birth occurred in 4.38% of unintended pregnancies, contrasted with 3.96% among intended pregnancies. Investigators calculated a risk ratio of 1.267 for preterm birth, indicating that roughly 21.1% of preterm deliveries within the study cohort could be attributed directly to pregnancy intention status.
Conversely, the target trial emulation found no direct association between unintended pregnancy and low birth weight. This outcome contrasted with results from a conventional adjusted regression analysis conducted alongside the study, which had flagged associations linking unintended pregnancy to both preterm birth and low infant birth weight. The researchers noted that methodological differences in managing confounding factors and handling missing outcome data likely accounted for the discrepancy between the two analytical approaches.
Mechanisms and Methodological Limitations
Several biological and behavioral pathways may explain the link between unintended pregnancy and shortened gestational duration. Elevated psychological stress and depression can trigger neuroendocrine and inflammatory pathways that influence uterine activity and cervical remodeling. Behavioral adjustments during pregnancy may also play a role.
The study carried distinct limitations. Pregnancy intention metrics were restricted to live births, meaning pregnancies resulting in miscarriage, stillbirth, or elective abortion were excluded from the dataset. Data regarding birth weight were frequently absent, and gestational age was recorded in months rather than precise weeks. A sensitivity analysis limited strictly to each woman’s most recent birth showed that the association with preterm birth lost statistical significance. Despite these caveats, the authors emphasize that expanding access to effective contraception, targeted reproductive health education, and comprehensive maternal healthcare can mitigate the health burdens linked to unintended pregnancy in vulnerable regions.
Related reading