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Biological factor, barrier 3 of 6

Birth weight and prenatal health

Low birth weight raises the risk of infant death and disability, removing potential future contributors before any ability can develop. Whether it also lowers adult IQ and earnings is disputed.

Evidence

  • Scale. In 2020, 19.8 million newborns (14.7% of live births) weighed under 2,500 grams, down from 16.6% in 2000. South Asia accounted for 44%. Birth weight data were missing for 28.7% of newborns, so the global figure is partly modeled (Okwaraji et al., 2024).
  • Twin studies. Black, Devereux, and Salvanes (2007) found that among Norwegian twins the lighter twin had lower adult height, IQ, education, and earnings. Studies in the United States and Sweden found effects on education and income, though the U.S. effects were generally small (Royer, 2009, Bharadwaj et al., 2018).
  • Danish study. Using a pregnancy complication that causes early birth, about two million births from 1981 to 2013 confirmed causal effects on infant death, health, and disability but found no strong evidence for effects on test scores, income, or IQ. Infant death effects have weakened as newborn care has improved (Maruyama and Heinesen, 2020).

What can be done

Actions rated on the strength-rating scale (A strong and replicated, B solid but limited, C weak or debated, D contested or failed), applied to the specific claim made.

  • Antenatal corticosteroids for threatened early preterm birth, in hospitals that can assess gestational age and care for newborns (A). A Cochrane review of 27 trials with 11,272 women found 22% fewer newborn deaths (McGoldrick et al., 2020), and the WHO ACTION-I trial in five low- and middle-income countries cut newborn deaths from 23.5% to 19.6% (WHO ACTION Trials Collaborators, 2020). Where the setting matters: a strategy that promoted the drug at all levels of care, including where gestational age could not be assessed, was followed by higher overall newborn mortality (RR 1.12), although not in the low birth weight babies it targeted (Althabe et al., 2015).
  • Kangaroo mother care, starting immediately after birth (A). Continuous skin-to-skin contact cut mortality at discharge by 40% across trials (Conde-Agudelo and Díaz-Rossello, 2016). In a WHO trial of 3,211 babies weighing 1.0 to 1.799 kg, starting it immediately cut 28-day deaths from 15.7% to 12.0% (WHO Immediate KMC Study Group, 2021). A Ugandan trial that started it before babies were clinically stable found no clear mortality benefit, so timing and setting matter (Tumukunde et al., 2024).
  • Multiple micronutrient tablets in pregnancy instead of iron and folic acid alone (A for low birth weight). Across 20 trials with 141,849 women, low birth weight fell 12%, with no effect on newborn deaths (Keats et al., 2019). A model estimated about $21 per healthy life-year gained in Bangladesh (Kashi et al., 2019).
  • Help to stop smoking in pregnancy (A for quitting, B for birth weight). A 2026 Cochrane review of 127 studies with 47,361 participants found 41% more women quit by late pregnancy, financial incentives roughly doubled quitting compared with other approaches, and low birth weight fell 18% (low-certainty evidence) (Freijah et al., 2026).
  • Malaria prevention in pregnancy in endemic areas (A for three or more doses of preventive treatment, B for bed nets). Three or more doses of sulfadoxine-pyrimethamine cut low birth weight by 20% compared with two (Kayentao et al., 2013), and replacing it with dihydroartemisinin-piperaquine alone did worse (Madanitsa et al., 2023). Insecticide-treated nets cut low birth weight by 23% in older African trials (Gamble et al., 2006).
  • Protein and energy supplements for undernourished pregnant women (B). Stillbirths fell 40% and small-for-age births fell 21% (Ota et al., 2015).
  • Enough antenatal visits (B). Cutting the number of visits raised perinatal deaths in low- and middle-income countries (RR 1.15), which supports WHO's eight-contact model as a platform for the other actions (Dowswell et al., 2015).
  • Chlorhexidine cord care (B for home births in high-mortality South Asian settings only). It cut newborn deaths by 19% in community trials, all in South Asia (Sinha et al., 2015), but two large African trials found no benefit (Semrau et al., 2016, Sazawal et al., 2016).
  • Not effective for birth weight: intravenous iron. A Malawi trial of 862 women improved anemia but not birth weight (Pasricha et al., 2023).
  • Combined effect (B, a model). Scaling eight proven antenatal actions in 81 countries could prevent an estimated 5.2 million small or vulnerable births a year, at about $1.1 billion in 2030 including two delivery-room actions (Hofmeyr et al., 2023).

Proposed and experimental methods

Methods that are proposed, under trial, approved in some places, or tried and then failed. Each shows a stage label and an evidence rating. A stage label shows how far a method has progressed, not whether it works. The stage labels are explained on the biological factor page.

  • Single-dose azithromycin in labor (Large trial, A for maternal sepsis, no newborn benefit). In the A-PLUS trial of 29,278 women planning vaginal birth in low- and middle-income countries, one 2 g oral dose cut maternal sepsis or death from 2.4% to 1.6% (relative risk 0.67), but stillbirth, newborn death, or sepsis did not change (10.5% versus 10.3%) (Tita et al., 2023). A 2025 Cochrane review of four trials with 42,846 women found maternal sepsis of 1.2% versus 1.8% with placebo (risk ratio 0.65, moderate certainty), no difference in newborn sepsis or death, and very uncertain evidence on antibiotic resistance (Suzuki et al., 2025).
  • Maternal RSV vaccination (Approved but not scaled, B). Vaccinating pregnant women passes antibodies to the baby against respiratory syncytial virus. In a Pfizer-funded phase 3 trial in 18 countries with 7,358 women, efficacy against severe medically attended RSV lung illness was 81.8% within 90 days and 69.4% within 180 days (Kampmann et al., 2023). The FDA approved it in August 2023 only for 32 through 36 weeks of pregnancy because preterm births were 5.7% versus 4.7% with placebo, and required further safety studies (FDA, 2023a), and a similar GSK vaccine trial was stopped early after preterm births reached 6.8% versus 4.9% (relative risk 1.37) (Dieussaert et al., 2024). WHO recommended in May 2025 that all countries introduce either the maternal vaccine or an infant antibody, noting that 97% of infant RSV deaths occur in low- and middle-income countries (WHO, 2025).
  • AI reading of blind ultrasound sweeps to date pregnancies (Early trial, B). A neural network estimates gestational age from simple sweeps of a probe across the abdomen, so staff without sonography training can date pregnancies with low-cost devices. In 4,695 pregnancies in North Carolina and Zambia, the average error was 3.9 days versus 4.7 days for standard measurement (Pokaprakarn et al., 2022). In 400 pregnancies, novice users with a battery-powered device were as accurate as credentialed sonographers (3.2 versus 3.0 days) (Stringer et al., 2024), and a separate Google-led model worked in Chicago and Nairobi (4.2 versus 4.5 days) (Willis et al., 2026). No trial has yet shown that AI dating improves birth outcomes.
  • Low-dose aspirin for all first pregnancies (Large trial, C). In 11,976 first-time mothers in six low- and middle-income countries, daily 81 mg aspirin started at 6 to 13 weeks cut preterm birth from 13.1% to 11.6% (relative risk 0.89) and lowered perinatal deaths (relative risk 0.86) (Hoffman et al., 2020). A meta-analysis of seven trials with 29,029 first-time mothers found no significant drop in preterm birth before 37 weeks (relative risk 0.96), a drop before 34 weeks (relative risk 0.84), and more bleeding after birth (relative risk 1.32) (Yan et al., 2024). The trial also required early ultrasound dating, which many settings lack.
  • Artificial womb for extremely premature babies (Theoretical, C). A fluid-filled system connected through the umbilical vessels would let an extremely premature baby keep developing as a fetus outside the uterus. Lambs at a stage equivalent to extremely premature human infants were supported for up to 4 weeks with stable circulation, normal blood gases, growth, lung maturation, and brain myelination (Partridge et al., 2017). An FDA advisory committee met in September 2023 to discuss first-in-human study design and ethics (FDA, 2023b), and in December 2024 the developer had raised $50 million but had not tested the system in humans (McCook, 2024). Searches up to October 8, 2026 found no report that a human trial has started.

Sources cited on this page

  1. Black, S. E., Devereux, P. J., & Salvanes, K. G. (2007). From the cradle to the labor market? The effect of birth weight on adult outcomes. Quarterly Journal of Economics, 122(1), 409-439. DOI B Moderate: strong for health effects, contested for IQ and earnings
  2. Royer, H. (2009). Separated at girth: US twin estimates of the effects of birth weight. American Economic Journal: Applied Economics, 1(1), 49-85. AEA B Moderate
  3. Bharadwaj, P., Lundborg, P., & Rooth, D.-O. (2018). Birth weight in the long run. Journal of Human Resources, 53(1), 189-231. JHR B Moderate
  4. WHO: Congenital disorders fact sheet (2023) B Moderate
  5. Maruyama, S., & Heinesen, E. (2020). Another look at returns to birthweight. Journal of Health Economics, 70, 102269. DOI A Strong: for mortality counts, B for causal estimates
  6. 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. Lancet, 403(10431), 1071-1080. Lancet01198-4/fulltext) B Moderate
  7. WHO: Nearly 1.8 billion adults not doing enough physical activity (2024) B Moderate: agency estimate
  8. Althabe, F., Belizán, J. M., McClure, E. M., Hemingway-Foday, J., Berrueta, M., Mazzoni, A., et al. (2015). A population-based, multifaceted strategy to implement antenatal corticosteroid treatment versus standard care for the reduction of neonatal mortality due to preterm birth in low-income and middle-income countries: The ACT cluster-randomised trial. The Lancet, 385(9968), 629-639. link (online October 2014) B Moderate
  9. Conde-Agudelo, A., & Díaz-Rossello, J. L. (2016). Kangaroo mother care to reduce morbidity and mortality in low birthweight infants. Cochrane Database of Systematic Reviews, 2016(8), CD002771. link A Strong
  10. Dowswell, T., Carroli, G., Duley, L., Gates, S., Gülmezoglu, A. M., Khan-Neelofur, D., & Piaggio, G. (2015). Alternative versus standard packages of antenatal care for low-risk pregnancy. Cochrane Database of Systematic Reviews, 2015(7), CD000934. link B Moderate
  11. Freijah, I., Jones, K. A., Coleman, T., Perlen, S. M., Kennedy, M., Passey, M. E., et al. (2026). Psychosocial interventions for supporting women to stop smoking in pregnancy. Cochrane Database of Systematic Reviews, 2026(4), CD001055. link A Strong: for abstinence, B for low birth weight
  12. Gamble, C., Ekwaru, J. P., & ter Kuile, F. O. (2006). Insecticide-treated nets for preventing malaria in pregnancy. Cochrane Database of Systematic Reviews, 2006(2), CD003755. link B Moderate
  13. Hofmeyr, G. J., Black, R. E., Rogozińska, E., Heuer, A., Walker, N., Ashorn, P., 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. link B Moderate
  14. Kashi, B., Godin, C. M., Kurzawa, Z. A., Verney, A. M. J., Busch-Hallen, J. F., & De-Regil, L. M. (2019). Multiple micronutrient supplements are more cost-effective than iron and folic acid: Modeling results from 3 high-burden Asian countries. The Journal of Nutrition, 149(7), 1222-1229. link B Moderate
  15. Kayentao, K., Garner, P., van Eijk, A. M., Naidoo, I., Roper, C., Mulokozi, A., et al. (2013). Intermittent preventive therapy for malaria during pregnancy using 2 vs 3 or more doses of sulfadoxine-pyrimethamine and risk of low birth weight in Africa: Systematic review and meta-analysis. JAMA, 309(6), 594-604. link A Strong
  16. Keats, E. C., Haider, B. A., Tam, E., & Bhutta, Z. A. (2019). Multiple-micronutrient supplementation for women during pregnancy. Cochrane Database of Systematic Reviews, 2019(3), CD004905. link A Strong
  17. Madanitsa, M., Barsosio, H. C., Minja, D. T. R., Mtove, G., Kavishe, R. A., Dodd, J., et al. (2023). Effect of monthly intermittent preventive treatment with dihydroartemisinin-piperaquine with and without azithromycin versus monthly sulfadoxine-pyrimethamine on adverse pregnancy outcomes in Africa: A double-blind randomised, partly placebo-controlled trial. The Lancet, 401(10381), 1020-1036. link B Moderate
  18. McGoldrick, E., Stewart, F., Parker, R., & Dalziel, S. R. (2020). Antenatal corticosteroids for accelerating fetal lung maturation for women at risk of preterm birth. Cochrane Database of Systematic Reviews, 2020(12), CD004454. link A Strong
  19. Ota, E., Hori, H., Mori, R., Tobe-Gai, R., & Farrar, D. (2015). Antenatal dietary education and supplementation to increase energy and protein intake. Cochrane Database of Systematic Reviews, 2015(6), CD000032. link B Moderate
  20. Pasricha, S.-R., Mwangi, M. N., Moya, E., Ataide, R., Mzembe, G., Harding, R., et al. (2023). Ferric carboxymaltose versus standard-of-care oral iron to treat second-trimester anaemia in Malawian pregnant women: A randomised controlled trial. The Lancet, 401(10388), 1595-1609. link B Moderate
  21. Sazawal, S., et al. (2016). Efficacy of chlorhexidine application to umbilical cord on neonatal mortality in Pemba, Tanzania: A community-based randomised controlled trial. Lancet Global Health, 4(11), e837-e844. link B Moderate
  22. Semrau, K. E. A., et al. (2016). Effectiveness of 4% chlorhexidine umbilical cord care on neonatal mortality in Southern Province, Zambia (ZamCAT): A cluster-randomised controlled trial. Lancet Global Health, 4(11), e827-e836. link B Moderate
  23. Sinha, A., Sazawal, S., Pradhan, A., Ramji, S., & Opiyo, N. (2015). Chlorhexidine skin or cord care for prevention of mortality and infections in neonates. Cochrane Database of Systematic Reviews, 2015(3), CD007835. link B Moderate: for South Asia, D if applied to sub-Saharan Africa
  24. Tumukunde, V., Medvedev, M. M., Tann, C. J., et al. (2024). Effectiveness of kangaroo mother care before clinical stabilisation versus standard care among neonates at five hospitals in Uganda (OMWaNA): A parallel-group, individually randomised controlled trial and economic evaluation. The Lancet, 403(10443), 2520-2532. link B Moderate
  25. WHO ACTION Trials Collaborators (Oladapo, O. T., Vogel, J. P., Piaggio, G., Nguyen, M. H., Althabe, F., Gülmezoglu, A. M., et al.). (2020). Antenatal dexamethasone for early preterm birth in low-resource countries. New England Journal of Medicine, 383(26), 2514-2525. link B Moderate
  26. WHO Immediate KMC Study Group (Arya, S., Naburi, H., Kawaza, K., Newton, S., Anyabolu, C. H., Bergman, N., et al.). (2021). Immediate "kangaroo mother care" and survival of infants with low birth weight. New England Journal of Medicine, 384(21), 2028-2038. link B Moderate
  27. Dieussaert, I., Hyung Kim, J., Luik, S., Seidl, C., Pu, W., Stegmann, J.-U., et al. (2024). RSV prefusion F protein-based maternal vaccine: Preterm birth and other outcomes. New England Journal of Medicine, 390(11), 1009-1021. link B Moderate
  28. Hoffman, M. K., Goudar, S. S., Kodkany, B. S., Metgud, M., Somannavar, M., Okitawutshu, J., et al. (2020). Low-dose aspirin for the prevention of preterm delivery in nulliparous women with a singleton pregnancy (ASPIRIN): A randomised, double-blind, placebo-controlled trial. The Lancet, 395(10220), 285-293. link B Moderate
  29. Kampmann, B., Madhi, S. A., Munjal, I., Simões, E. A. F., Pahud, B. A., Llapur, C., et al. (2023). Bivalent prefusion F vaccine in pregnancy to prevent RSV illness in infants. New England Journal of Medicine, 388(16), 1451-1464. link B Moderate
  30. McCook, A. (2024, December 4). An artificial womb for preemies? Philly company gets $50 million to pursue. The Philadelphia Inquirer. link B Moderate
  31. Partridge, E. A., Davey, M. G., Hornick, M. A., McGovern, P. E., Mejaddam, A. Y., Vrecenak, J. D., et al. (2017). An extra-uterine system to physiologically support the extreme premature lamb. Nature Communications, 8, 15112. link C Limited: B for lambs
  32. Pokaprakarn, T., Prieto, J. C., Price, J. T., Kasaro, M. P., Sindano, N., Shah, H. R., et al. (2022). AI estimation of gestational age from blind ultrasound sweeps in low-resource settings. NEJM Evidence, 1(5), EVIDoa2100058. link B Moderate
  33. Stringer, J. S. A., Pokaprakarn, T., Prieto, J. C., Vwalika, B., Chari, S. V., Sindano, N., et al. (2024). Diagnostic accuracy of an integrated AI tool to estimate gestational age from blind ultrasound sweeps. JAMA, 332(8), 649-657. link B Moderate
  34. Suzuki, D., Yamaji, N., Nishimura, E., Suzuki, H., Ishikawa, K., Rahman, M. O., et al. (2025). Effects of antibiotic prophylaxis during labour on maternal and neonatal outcomes in women planning vaginal birth. Cochrane Database of Systematic Reviews, 2025(8), CD016211. link A Strong
  35. Tita, A. T. N., Carlo, W. A., McClure, E. M., Mwenechanya, M., Chomba, E., Hemingway-Foday, J. J., et al. (2023). Azithromycin to prevent sepsis or death in women planning a vaginal birth. New England Journal of Medicine, 388(13), 1161-1170. link B Moderate
  36. Willis, A., Lee, C., Krogue, J., Wickramanayake, A., Young-Lin, N., Caron, S., et al. (2026). Generalization of AI-based gestational age assessment using blind sweep ultrasonography. JAMA Network Open, 9(7), e2622484. link B Moderate
  37. Yan, X., Zheng, W., Wang, J., Yuan, X., & Li, G. (2024). Low-dose aspirin for the prevention of preterm birth in nulliparous women: Systematic review and meta-analysis. BMC Pregnancy and Childbirth, 24(1), 260. link B Moderate

Every source for this factor is listed on the biological factor page.