🟠 Moderate Evidence
A 15-year longitudinal study of over 1,200 UK mother-child pairs has identified a specific cognitive gap linked to prenatal iodine deficiency: children whose mothers had severe iodine insufficiency in the first trimester scored approximately 4 points lower on verbal intelligence tests at age 15 compared to children of iodine-sufficient mothers, according to research published in the European Journal of Nutrition in March 2026. Notably, abstract reasoning and non-verbal intelligence remained unaffected, suggesting iodine’s role in language development may be distinct from other cognitive domains.
Key takeaways
- Severe maternal iodine deficiency (below 50 µg/g creatinine) in first trimester associated with 4-point lower Vocabulary T-score at age 15
- The deficit was specific to verbal intelligence; abstract reasoning and Matrix Reasoning scores were unaffected
- Researchers attributed the timing to fetal reliance on maternal thyroid hormone before fetal thyroid autonomy at 16–20 weeks gestation
- Many prenatal vitamin products in the UK contain little or no iodine despite evidence of its cognitive importance
Study at a Glance
| Source | European Journal of Nutrition |
| Study type | Prospective cohort analysis with 15-year follow-up |
| Sample size | N = 1,211 mother-child pairs |
| Population | Pregnant women enrolled in first trimester; children tested at age 15 |
| Country | United Kingdom |
Maternal Iodine Status and Child Vocabulary Score at Age 15
Vocabulary T-score decline by first-trimester maternal urinary iodine level, ALSPAC cohort, UK
Source: Keestra et al., European Journal of Nutrition, 2026 | Georgian Medical Journal News
Iodine and the Fetal Brain: A Critical Window
The timing of iodine’s impact is critical to understanding why first-trimester maternal status matters so much. The fetal brain begins its development before the fetus has a functioning thyroid gland—a process called organogenesis that is particularly dependent on thyroid hormone. According to the analysis conducted by Keestra and colleagues in the European Journal of Nutrition, the fetal thyroid does not reach functional autonomy until approximately 16 to 20 weeks of gestation.
Before that milestone, the developing fetus relies entirely on maternal thyroxine (T4) crossing the placenta to supply the thyroid hormone needed for neural development. Without sufficient iodine—the essential micronutrient required to synthesize thyroid hormone—the mother cannot produce adequate T4, and fetal brain development is compromised. This explains why the association with verbal intelligence, which has strong developmental roots in early cortical language networks, appears more specific than effects on abstract reasoning.
The study’s data were drawn from the Avon Longitudinal Study of Parents and Children (ALSPAC), one of the world’s largest pregnancy cohorts. Researchers measured urinary iodine-to-creatinine ratios in first-trimester maternal urine samples, then followed the cohort prospectively. At age 15, children completed the Two-Subtest Wechsler Abbreviated Scale of Intelligence (WASI), which yields separate scores for Vocabulary (verbal intelligence) and Matrix Reasoning (abstract, non-verbal intelligence). The linear regression analysis adjusted for multiple confounders.
Vocabulary Targeted, Reasoning Spared
One of the study’s most striking findings is its specificity. Children of mothers in the severe iodine deficiency group (urinary iodine below 50 µg/g creatinine) scored approximately 4 T-score points lower on the Vocabulary subtest at age 15, compared to the iodine-sufficient reference group (150–250 µg/g creatinine). Full-Scale IQ also declined by about 3 points at severe deficiency, but this reduction was mathematically driven entirely by the Vocabulary component—Matrix Reasoning scores held steady across all iodine strata.
This pattern did not differ by sex. The specificity of the effect suggests that iodine’s role in prenatal neurodevelopment may be particularly important for language and verbal processing networks, while leaving other cognitive domains—such as visuo-spatial and abstract reasoning—relatively intact. This selective vulnerability is consistent with animal and human evidence showing that thyroid hormone is critical for myelination and dendritic development in the temporal and frontal language areas during the prenatal period.
A notable gap exists between evidence and practice: as reported in the source analysis, many prenatal vitamin formulations available in the UK contain little or no iodine, despite National Institute for Health and Care Excellence (NICE) and WHO guidance recognizing iodine as essential for pregnancy. This represents a potential public health opportunity to align product formulation with evidence.
For more on clinical updates on prenatal care, see our ongoing coverage.
Children of mothers with severe first-trimester iodine deficiency (below 50 µg/g creatinine) scored approximately 4 T-score points lower on verbal intelligence at age 15 than children of iodine-sufficient mothers, while abstract reasoning remained unaffected.
— Keestra et al., European Journal of Nutrition (March 2026)
Implications for Prenatal Care and Public Health
The ALSPAC findings add to a growing body of evidence that iodine supplementation during pregnancy should be non-negotiable, not optional. The World Health Organization (WHO) has long recognized iodine deficiency as one of the most common preventable causes of intellectual disability in children worldwide. Yet in high-income countries with seemingly robust micronutrient surveillance, iodine deficiency remains underappreciated and under-treated in pregnancy.
The study suggests that the first trimester is a critical window—potentially more sensitive than later pregnancy—for ensuring adequate maternal iodine status. Standard prenatal iodine supplementation protocols typically recommend 150 µg daily for pregnant women, though some guidelines recommend up to 200–250 µg. This research underscores the importance of universal supplementation rather than risk-based approaches, since iodine insufficiency is often asymptomatic and urinary iodine-to-creatinine ratios are not routinely measured in standard antenatal screening in many countries.
For further insight into pregnancy-related research, see our coverage of new studies in maternal health.
What this means
Frequently asked questions
Is iodine deficiency common in pregnant women in the UK and high-income countries?
Yes. Although the UK is not classified as severely iodine-deficient overall, population surveys show that a significant proportion of women of childbearing age have urinary iodine levels below WHO adequacy thresholds, particularly in those who avoid dairy and do not use iodized salt. The ALSPAC cohort itself demonstrated variation in iodine status across participants, with some in the severe deficiency range, indicating that risk exists even in wealthy healthcare systems.
Does iodine supplementation need to start before pregnancy, or is first-trimester supplementation sufficient?
While the ALSPAC study measured first-trimester iodine status and found associations with later cognitive outcomes, evidence suggests that iodine stores and maternal thyroid hormone production are important throughout pregnancy. Current guidelines recommend iodine supplementation begin ideally before conception and continue throughout pregnancy and lactation. The critical window for fetal brain development spans much of the second and third trimesters as well as the first trimester.
Can pregnant women get enough iodine from diet alone?
In regions with iodized salt or adequate dietary iodine from seafood and dairy, dietary sources may provide sufficient iodine. However, in areas with low iodine soil content, or among women avoiding salt or consuming plant-based diets, dietary intake alone may be insufficient. Supplementation via prenatal vitamins is recommended as a reliable, cost-effective strategy to ensure adequacy, rather than relying on dietary assessment.
The ALSPAC cohort continues to provide invaluable long-term data on the life-course determinants of health and development. This latest analysis on iodine and cognition reinforces what basic neuroscience has long suggested: micronutrient status in early pregnancy is not a minor detail, but a foundational pillar of child development. As prenatal vitamin formulations are revised and updated, iodine should move from the margins to the center of attention—right where folate already stands. For ongoing coverage of global health research and micronutrient science, stay tuned to GMJ News.
Source: Folate gets the lion's share of attention in prenatal vitamins
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Medically reviewed by Prof. Giorgi Pkhakadze, MD, MPH, PhD. Spotted an error? Contact the editorial team.
