Purpose: To assess the following in singleton pregnant women: 1) associations between first trimester iron deficiency and obstetric and perinatal outcomes, 2) overall first trimester iron status and 3) post-treatment iron status after intensified iron supplementation.

Methods: Prospective cohort study with linkage of first trimester hemoglobin and plasma ferritin with obstetric and perinatal data from a hospital database. Blood sample data were obtained at a Danish University Hospital. The cohort was divided into groups according to ferritin and hemoglobin: (1) iron deficient anemic (ferritin <30 ng/mL and Hb <110 g/L), (2) iron deficient non-anemic (ferritin <30 ng/mL and Hb ≥110 g/L), and (3) iron replete non-anemic (ferritin 30–200 ng/mL and Hb ≥110 g/L). Obstetric and perinatal outcomes in each iron deficient group were compared to the iron replete non-anemic group using multivariable logistic regression. The effect of 2–8 weeks intensified iron supplementation on hemoglobin and ferritin were assessed by pairwise comparisons.

Results: The cohort comprised 5,763 singleton pregnant women, of which 14.2% had non-anemic iron deficiency, and 1.2% had iron deficiency anemia. Compared to iron replete non-anemic women, iron deficient anemic women had a higher risk of gestational diabetes (aOR 3.8, 95% CI 1.4–9.0), and iron deficient non-anemic women had a higher risk of stillbirth (aOR 4.0, 95% CI 1.0–14.3). In group 1 and 2, 78.7% and 67.8% remained iron deficient after intensified iron supplementation.

Conclusion: First trimester iron deficiency may be associated with gestational diabetes and stillbirth. First trimester iron deficiency was present in 15.4% and often persisted despite 2–8 weeks intensified iron supplementation.