Iodine is essential in foetal development through being an integral part of the thyroid hormones. Severe iodine deficiency is associated with foetal growth restriction and preterm delivery. Less is known about the potential impact of mild-to-moderate iodine deficiency on these outcomes.

The aim of this study was to investigate whether maternal iodine intake in pregnancy was associated with birth weight (BW) z-score (i.e. BW adjusted for gestational length and sex) and preterm delivery (before week 37).

The study population included 77,995 singleton pregnancies from The Norwegian Mother and Child Cohort Study recruited in gestational week 15 in the period 2002–2008. Habitual iodine intake was calculated from a validated food frequency questionnaire covering the first half of pregnancy. Use of supplements was reported in questionnaires. Urinary iodine concentration (UIC) was measured in gestational week 18 in a subsample of n = 2795 women. Median iodine intake from food was 121 μg/day and median UIC was 69 μg/L. Median UIC < 150 μg/L is considered insufficient in pregnant women. Median birthweight was 3610 g and 5.0% were born before gestational week 37. Associations were modelled flexibly by use of restricted cubic splines, and adjusted for age, parity, pre-pregnancy BMI, education, smoking in pregnancy, energy intake, and fibre intake.

In non-users of iodine-containing supplements (n = 48,958), a low habitual iodine intake from food (lower than about 150 μg/day) was associated with a lower mean BW z-score (p < 0.001). Compared to an intake of 150 μg/day (reference), mean z-score was 0.04 SD lower at 100 μg/day and 0.12 SD lower at 75 μg/day. Results were similar when using UIC as the exposure (n = 2795, p = 0.017). Any use of iodine containing supplements in pregnancy was associated with 0.03 (95% CI: 0.01, 0.04) SD increase in BW z-score compared to no use (n = 77,949, p < 0.001).

A low habitual iodine intake from food (lower than about 100 μg/day) was associated with increased risk of preterm delivery (p = 0.003). Compared to an intake of 100 μg/day (reference), 75 μg/day was associated with 10% increased risk, and 50 μg/day with 28% increased risk. Use of an iodine-containing supplement was not associated with the risk of preterm delivery (OR: 0.97 (95%CI: 0.91, 1.04, p = 0.42)).

Inadequate iodine intake is prevalent in women of childbearing age in otherwise well-nourished populations. Our results indicate that mild-to-moderate iodine deficiency in pregnancy is associated with restricted foetal growth and increased risk of preterm delivery.