Low birth weight, prematurity and growth retardation: small vulnerable newborns
- Page updated onJune 29, 2026
Approximately, for every four live births, one falls into the category of small and vulnerable newborns, which includes preterm, small for gestational age, and those who meet both conditions simultaneously. These three subcategories encompass, practically, the totality of what has traditionally been known as "low birth weight newborns", but with more clarity as to the causal pathways of their vulnerability.
The problem, of course, most affects newborns whose mothers experience pregnancy exposed to malnutrition, gender-based violence, disease, poverty and conflict. The consequences are experienced for years, when those who have not suffered premature death live at greater risk of wasting or stunting, illness, delayed development, and as a consequence have fewer opportunities for education, employment and to escape the cycle of intergenerational poverty.
Table of contents:
What are small vulnerable newborns?
The term includes babies who are too small and those who are born too early.
Often, adverse conditions experienced by some infants during the intrauterine period cause:
- Preterm births before 37 weeks of gestation (around 8.8% of all live newborns).
- At term but small below the 10th percentile for their gestational age (around 16.3%), as a consequence of intrauterine fetal growth retardation.
- Being born with both problems simultaneously: premature and also small for gestational age (around 1.1%) (Lawn et al., 2023).
These three categories allow a much larger group of at-risk newborns to be identified, greater than that traditionally grouped under the term low birthweight newborn. The new terminology, introduced in 2023, makes it possible to go beyond categorisation based on a relatively arbitrary threshold of 2,500g birthweight (UNICEF & WHO, 2019), introduced around 1918 and formalised by the WHO in 1950 (Hughes et al., 2017). The new category of vulnerable newborns now also includes premature or small-for-age babies who, despite being born above that weight threshold, face similar risks to their lives. The focus is now also placed on the different underlying biological causes of the problem and on how to address each of them through prevention and clinical management.
It is estimated (with 2020 data) that 14.7% of all live births are low birthweight (<2,500g), equivalent to 1 in 7 (Okwaraji et al., 2024). This percentage exceeds 15% in places such as South and West Africa, and is even above 20% in India or the Philippines (UNICEF, 2023).
What are the consequences of the vulnerability of small newborns?
Small and vulnerable newborns have a high risk of neonatal mortality. It is estimated that 32.8% of neonatal deaths are due to preterm birth, 14.7% to being small for gestational age and 7.7% to the combination of both conditions, totalling 55.3% of neonatal deaths (Lawn et al., 2023).
In addition to neonatal deaths from this cause, fetal deaths must also be considered, since many small vulnerable babies do not survive to be born alive. The problems that can lead to preterm birth or low birthweight are the same ones that increase the risk of fetal death. Despite this, fetal mortality, which is largely preventable, has barely received attention or been considered a public health priority. It is often overlooked and excluded from global indicators and targets that are calculated solely on the basis of live births (Lawn et al., 2023).
Small and vulnerable newborns who survive the neonatal period also face a lifelong risk of health and developmental problems. Among the consequences are growth problems and a greater risk of malnutrition, delayed cognitive development and learning difficulties, a higher risk of childhood illnesses (such as asthma, epilepsy, behavioural problems, and vision and hearing problems), or chronic diseases in adult life such as hypertension, cardiovascular problems, type 2 diabetes or metabolic syndrome. All of this, of course, ultimately impacts their education, employment opportunities, social success and income (Ashorn et al., 2023).
Known causes of prematurity and low birth weight: malnutrition, infections and environmental factors
Why do cases of small vulnerable newborns occur?
Although the mechanisms leading to a small and vulnerable newborn differ, their causes overlap. Among them are contextual factors such as poverty, conflict, low maternal education (Mahumud et al., 2017), health system deficiencies, environmental pollution and gender inequality. These factors increase exposure to the immediate causes of preterm birth and intrauterine growth restriction. Three of them account for almost half of all cases: maternal malnutrition, infections during pregnancy and environmental factors (Hunter et al., 2023).
Maternal malnutrition may be related to insufficient reserves of nutrients and energy for proper foetal development and growth, and can be diagnosed through low weight and height, low BMI or reduced mid-upper arm circumference (Kapil & Ververs, 2023). However, it can also manifest as micronutrient deficiencies such as iron, zinc and calcium. These influence the functioning of enzymes and hormones involved in foetal growth, placental function, the balance of hormones related to the onset of labour, maternal immunity, blood pressure (and the risk of pre-eclampsia), foetal oxygenation and bone formation.
Among the main infections that result in small vulnerable newborns are malaria, HIV/AIDS and sexually transmitted infections. These can affect the placenta, cause anemia, produce hormonal imbalances, worsen the mother's nutritional status, cross the placental barrier and infect the fetus, and even trigger premature delivery.
Environmental factors include air pollution, intimate partner violence, physical overload, and alcohol and tobacco use. These affect the fetus and placenta through oxidative stress, physical and mental trauma, reduced uterine blood flow, premature uterine contractions, and placental toxicity.
Many cases of small vulnerable newborns can be prevented with prenatal care
As the mechanisms leading to the birth of small and vulnerable newborns become better understood, so too does their prevention.
Eight interventions are recommended that can be offered throughout antenatal care. These are multiple micronutrient supplements, protein-energy supplements, aspirin administration, treatment of syphilis cases, smoking cessation education, malaria prevention, treatment of asymptomatic bacteriuria, and vaginal progesterone administration.
Alongside these, there are three other interventions that appear to have great potential, although they are still being researched. These are the administration of omega-3 fatty acid supplements, calcium and zinc (Hofmeyr et al., 2023).
If implemented with the recommended coverage, the implementation of the first eight prenatal interventions could reduce total low birth weight newborns by 17.9%, stillbirths by 31.5%, and neonatal deaths by 20%. If the other three potential interventions were added to these eight, the reduction in low birth weight cases could reach 28.6%.
When considering these interventions from a public health and population impact perspective, we should highlight two of them: multiple micronutrient supplementation, and protein-energy supplementation. These two interventions are the only ones with proven effect in reducing the cases of small-for-gestational-age term newborns. Moreover, these two interventions alone, together, would account for 67.1% of the effect of the eleven recommended interventions in preventing cases of small and vulnerable newborns, and up to 32% of their effect in reducing neonatal mortality.
Unfortunately, in humanitarian crises and resource-constrained settings, many women attend their first antenatal visit very late, receive none at all, or receive antenatal care of very poor quality, in which these evidence-based interventions are not available (Le & Nguyen, 2019).
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Integrated management of small vulnerable newborns: from Kangaroo mother care to the MAMI Care Pathway
Beyond prevention, there are a number of evidence-based recommendations for the clinical management of small and vulnerable newborns (McGrath, 2023). Among these, kangaroo mother care (WHO, 2025) and skin-to-skin contact with the mother (or another caregiver), exclusive breastfeeding wherever possible, and iron supplementation stand out, among other measures (WHO, 2022).
The MAMI Care Pathway approach responds to the needs of high-risk infants and their mothers.
Small and vulnerable newborns today are the children with acute and chronic malnutrition of tomorrow (Kerac et al., 2025b). For this reason, it is increasingly difficult to justify addressing their needs through separate and fragmented theoretical frameworks, approaches and medical and nutritional protocols (Kerac et al., 2025a).
In this regard, the management approach for infants under six months who are small and at nutritional risk along with their mothers (known as the MAMI Care Pathway) can serve as a bridge between the management of small and vulnerable newborns and the management of acute malnutrition in children under five (MAMI Global Network et al., 2021).
The MAMI Care Pathway approach, community-based and adapted to humanitarian contexts and resource-limited settings, focuses on the needs of the mother-infant dyad where the baby is small and at nutritional risk (in the first six months of life). It provides technical guidance to identify them as early as possible, assess their psychosocial, nutritional and health problems and risks, and provide comprehensive support for their survival and wellbeing (Desplats et al., 2026). This support is linked to the rest of the available systems, facilitating referral to health services when there are serious problems, and interventions addressing the mother's needs, such as micronutrient supplementation and mental health and psychosocial support interventions (Fuller et al., 2025).
References
- Ashorn, P., Ashorn, U., Muthiani, Y., et al. (2023). Small vulnerable newborns — big potential for impact. The Lancet, 401(10389), 1692–1706. https://doi.org/10.1016/S0140-6736(23)00354-9
- Desplats, G., Dent, N., Ben Ameur, A., & Kitamura, T. (2026). MAMI en pratique: Learning from the field [Technical brief]. Emergency Nutrition Network. https://doi.org/10.71744/z6j7-wa12
- Fuller, S., Dent, N., Deconinck, H., & McGrath, M. (2025). Supporting mothers of vulnerable infants under six months: Inviting conversation and collaboration. Emergency Nutrition Network. https://www.ennonline.net/resource/mami/supporting-mothers-vulnerable-infants-under-six-months-inviting-conversation
- Hofmeyr, G. J., Black, R. E., Rogozinska, E., et al. (2023). Evidence-based antenatal interventions to reduce the incidence of small vulnerable newborns and their associated poor outcomes. The Lancet, 401(10389), 1733–1744. https://doi.org/10.1016/S0140-6736(23)00355-0
- Hughes, M. M., Black, R. E., & Katz, J. (2017). 2500-g low birth weight cutoff: History and implications for future research and policy. Maternal and Child Health Journal, 21(2), 283–289. https://doi.org/10.1007/s10995-016-2131-9
- Hunter, P. J., Awoyemi, T., Ayede, A. I., et al. (2023). Biological and pathological mechanisms leading to the birth of a small vulnerable newborn. The Lancet, 401(10389), 1720–1732. https://doi.org/10.1016/S0140-6736(23)00573-1
- Kapil, S., & Ververs, M. (2023). Maternal mid-upper arm circumference: Still relevant to identify adverse birth outcomes in humanitarian contexts? Field Exchange, 70. https://www.ennonline.net/fex/70/en/maternal-mid-upper-arm-circumference-still-relevant-identify-adverse-birth-outcomes
- Kerac, M., Ashorn, P., Berkley, J. A., et al. (2025a). Infants less than 6 months of age at risk of poor growth and development: Evidence gaps identified during WHO guideline development. BMJ Global Health, 10(Suppl. 5), e017227. https://doi.org/10.1136/bmjgh-2024-017227
- Kerac, M., James, P. T., McGrath, M., Brennan, E., Cole, T., Opondo, C., & Frison, S. (2025b). Malnutrition in infants aged under 6 months: Prevalence and anthropometric assessment — analysis of 56 low- and middle-income country DHS datasets. BMJ Global Health, 10(5), e016121. https://doi.org/10.1136/bmjgh-2024-016121
- Lawn, J. E., Ohuma, E. O., Bradley, E., et al. (2023). Small babies, big risks: Global estimates of prevalence and mortality for vulnerable newborns to accelerate change and improve counting. The Lancet, 401(10389), 1707–1719. https://doi.org/10.1016/S0140-6736(23)00522-6
- Le, K., & Nguyen, M. (2019). Armed conflict and birth weight (MPRA Paper No. 102162). University Library of Munich. https://mpra.ub.uni-muenchen.de/102162/
- Mahumud, R. A., Sultana, M., & Sarker, A. R. (2017). Distribution and determinants of low birth weight in developing countries. Journal of Preventive Medicine and Public Health, 50(1), 18–28. https://doi.org/10.3961/jpmph.16.087
- MAMI Global Network, Emergency Nutrition Network, & London School of Hygiene and Tropical Medicine. (2021). MAMI Care Pathway Package (Version 3). https://www.ennonline.net/mami/resources/en/mami-care-pathway-package-version-3-2021
- McGrath, M. (2023). The Lancet «Small Vulnerable Newborns» series: Reflections from a «MAMI» perspective. Field Exchange, 70. https://www.ennonline.net/fex/70/the-lancet-small-vulnerable-newborns-series-reflections-from-a-mami-perspective
- Okwaraji, Y. B., Krasevec, J., Bradley, E., et al. (2024). National, regional, and global estimates of low birthweight in 2020, with trends from 2000: A systematic analysis. The Lancet, 403(10431), 1071–1080. https://doi.org/10.1016/S0140-6736(23)01198-4
- UNICEF. (2023). Low birthweight. UNICEF Data. https://data.unicef.org/topic/nutrition/low-birthweight/
- UNICEF & WHO. (2019). UNICEF-WHO low birthweight estimates: Levels and trends 2000–2015. World Health Organization. https://www.unicef.org/reports/UNICEF-WHO-low-birthweight-estimates-2019
- World Health Organization. (2022). WHO recommendations for care of the preterm or low-birth-weight infant. World Health Organization. https://www.who.int/publications/i/item/9789240058262
- World Health Organization. (2025). Kangaroo mother care: A clinical practice guide. World Health Organization. https://www.who.int/publications/m/item/kangaroo-mother-care–a-clinical-practice-guide
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Abarca, B. (June 29, 2026). Low birth weight, prematurity and growth retardation: small vulnerable newborns. Salud Everywhere. https://saludeverywhere.com/en/health-in-humanitarian-crises/small-vulnerable-newborns/