Updated 25/05/2026
A 15-year follow-up study of over 1,200 UK mother-child pairs has revealed that severe iodine deficiency during the first trimester of pregnancy predicts lower verbal intelligence scores when children reach adolescence. The research, published in the European Journal of Nutrition, adds new evidence to concerns about iodine gaps in prenatal nutrition.
Maternal Iodine Status and Child Vocabulary Scores at Age 15
T-scores by first-trimester urinary iodine levels (µg/g creatinine)
Source: Keestra et al., European Journal of Nutrition, 2026 | Georgian Medical Journal News
Critical Window for Brain Development
The Avon Longitudinal Study of Parents and Children (ALSPAC), one of the world’s largest pregnancy cohorts, provided the data for this analysis by Keestra and colleagues. Researchers measured urinary iodine-to-creatinine ratios in first-trimester maternal urine from 1,211 mother-child pairs, then tracked the children’s cognitive development for 15 years.
At age 15, children underwent the Two-Subtest Wechsler Abbreviated Scale of Intelligence (WASI), which produces separate scores for Vocabulary (verbal intelligence) and Matrix Reasoning (abstract, non-verbal reasoning). The study, published in the European Journal of Nutrition in March 2026, found a specific pattern of cognitive impact.
Verbal Intelligence Shows Clear Dose-Response Pattern
Children whose mothers had severe iodine deficiency (urinary iodine below 50 µg/g creatinine) during the first trimester scored approximately 4 T-score points lower on Vocabulary tests at age 15 compared to children of iodine-sufficient mothers (150-250 µg/g), according to the Keestra et al. analysis. Full-Scale IQ was also about 3 points lower in the severe deficiency group.
Matrix Reasoning scores remained unaffected across all iodine status groups, according to the ALSPAC study, suggesting the impact is specific to verbal cognitive domains. The pattern did not differ by sex, as reported by Keestra and colleagues.
Thyroid Hormone Gap in Early Pregnancy
The fetal thyroid gland does not achieve functional autonomy until approximately 16-20 weeks of gestation, according to the original source material. During the first trimester, the developing fetus relies entirely on maternal thyroxine (T4) crossing the placenta to supply essential thyroid hormones for brain development.
Prenatal Supplement Gaps Persist
The World Health Organization recommends 250 µg daily iodine intake during pregnancy and lactation, according to WHO guidance.
Lower first-trimester maternal iodine status predicted lower Vocabulary T-scores at age 15, with children of severely deficient mothers scoring 4 points lower than those of iodine-sufficient mothers.
— Keestra et al., European Journal of Nutrition, March 2026
Key takeaways
- Severe maternal iodine deficiency (below 50 µg/g creatinine) in first trimester linked to 4-point lower verbal intelligence scores at age 15, according to Keestra et al.
- Impact specific to vocabulary and verbal reasoning; abstract reasoning abilities unaffected, per the ALSPAC analysis
- Critical period occurs before fetal thyroid becomes functional at 16-20 weeks gestation
- WHO recommends 250 µg daily iodine during pregnancy
Frequently asked questions
How much iodine should pregnant women consume daily?
The World Health Organization recommends 250 µg of iodine daily during pregnancy and lactation. This represents an increase from the 150 µg recommended for non-pregnant adults.
What did the ALSPAC study find about timing?
The Keestra et al. analysis suggests the most critical window is the first trimester, based on the 15-year follow-up data from 1,211 mother-child pairs.
The 15-year follow-up data from the ALSPAC cohort provides evidence that early pregnancy nutrition decisions have measurable consequences extending into adolescence, according to the Keestra et al. analysis.
Source: Keestra et al., European Journal of Nutrition, March 2026
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