UNINTENDED pregnancy may increase the risk of preterm birth, according to a large analysis of more than 900,000 pregnancies across 37 low- and middle-income countries.
Using a target trial emulation approach designed to strengthen causal inference from observational data, researchers found that unintended pregnancy was associated with a 26.7% increased risk of preterm birth.
Unintended pregnancies, defined as those that are mistimed or unwanted at conception, remain a major global reproductive health concern.
Researchers analysed Demographic and Health Surveys collected between 1997 and 2021 across 37 low- and middle-income countries, studying 911,423 pregnancies among 635,139 women aged 15 and 49 years. Of these pregnancies, 205,655, or 22.6%, were unintended.
Rather than relying solely on conventional observational analysis, the researchers used target trial emulation, an approach designed to approximate some features of a randomised controlled trial using observational data.
Statistical weighting was used to balance characteristics including age, BMI, education, household wealth, marital status, smoking, residence, reproductive history, and knowledge of family planning methods between intended and unintended pregnancies.
In the target trial emulation, unintended pregnancy was associated with a 26.7% increased risk of preterm birth compared with intended pregnancy.
In total, researchers found that preterm birth occurred in 4.38% of unintended pregnancies compared with 3.96% of intended pregnancies.
They estimated a risk ratio of 1.267 for preterm birth, with the analysis suggesting that approximately 21.1% of preterm births in the study population could be attributable to unintended pregnancy.
In contrast, unintended pregnancy was not associated with low birth weight in the target trial emulation.
This differed from a conventional adjusted regression analysis, which identified associations between unintended pregnancy and both preterm birth and low birth weight. The researchers suggest that differences in the methods used to account for confounding and missing outcome data could explain the discrepancy.
Several mechanisms could potentially contribute to the relationship between unintended pregnancy and preterm birth.
The researchers note that psychological stress and depression can influence neuroendocrine and inflammatory pathways involved in uterine activity and cervical changes. Behavioural factors may also contribute.
However, the study has limitations. Pregnancy intention was only available for live births, meaning pregnancies ending in miscarriage, stillbirth, or abortion were excluded. Birth weight data were also frequently missing, and gestational duration was recorded in months rather than weeks.
One sensitivity analysis restricted to each woman’s most recent birth also found that the association with preterm birth was no longer statistically significant.
Nevertheless, the researchers conclude that improving access to effective contraception, reproductive health education, and maternal healthcare could help reduce the health burden associated with unintended pregnancy, particularly in low- and middle-income countries.
Reference
Zou S, Rong X, Zhang M, et al. [Study of unintended pregnancy and adverse birth outcomes using target trial emulation]. 2026.
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