Objectives This study aimed to assess the impact that child marriage could have on the morbidity and mortality of children under 5 years, among a nationally representative sample of births to women aged 15-25 years in Afghanistan. Methods A secondary analysis of data from the 2015 Afghanistan Demographic and Health Survey (DHS), including the births in the past 5 years to the ever-married women aged 15-24 years old. Bivariate and multivariate analyses were employed to examine the association of child marriage (<18y) with morbidity (diarrhea, acute respiratory infection, and fever in the last 2 weeks), mortality (neonatal, infant, child), low birth weight (<2500g), and size at birth among the births to women aged 15-24 years, adjusting for the effect of sociodemographic and structural inequalities. Results Approximately two-thirds of births in the past 5 years were born to mothers aged <18 years. The majority of them were residing in rural areas (75.67%) and were born to illiterate mothers (51.68%) from poor families (39.39%). There was only a significant association between child marriage (<18) with neonatal mortality for births to the women married at ages <18 years (crude OR = 2.30, 95% CI: 1.52-3.49 & adjusted OR = 1.94, 95% CI: 1.25-3.01) and infant mortality for births to the women married at ages <=14y (crude OR = 1.94, 95% CI: 1.06-3.53). However, it disappeared for neonatal mortality after adjustment for antenatal care visits and infant mortality after adjustment for sociodemographic inequalities. Conclusion Although the births to women married as a child were more likely to die at an early age, this association disappeared after adjustment for the number of ANC visits. Given the inadequate ANC in child brides in Afghanistan is mainly due to a lack of autonomy and inability in decision-making to negotiate their reproductive, strong global advocacy is required to empower the young Afghan women by reducing the existent social and gender-based inequalities and reduce the consequent adverse health outcomes in both child mothers and offspring.

The authors have declared no competing interest.

The research was supported by Individual (O.D) Research Grants (Grant Number WU-IRG-65-017) from Walailak University, Thailand. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The data used in this study could be accessed upon a reasonable request from the DHS website and are publicly available. The ethical necessities for data collection were sought and assured by the institutions that commissioned, funded, or managed the surveys. All DHS are reviewed and approved by ICF International and Institutional Review Board of Ministry of Health in the target country to ensure that the protocols comply with the U.S. Department of Health and Human Services regulations for the protection of human subjects. Therefore, this study did not require further ethical approval.

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I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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The DHS questionnaire that collected the data in Afghanistan's demographic and health survey in 2015 could be downloaded from DHS official website (https://www.dhsprogram.com). The dataset (ADHS 2015) that was used in this study could be available upon a reasonable request and with permission from either the Walailak University ethical board or the DHS website.