A total of 1257 women had dietary recall information in the first trimester. The mean dietary iron intake from food was 11.5 mg/day (SD = 5.3), with only 20% (n= 257) of women reporting intake >14.8 mg/day [95% confidence interval (CI): 18, 23%]. Women who reported iron intake less than or equal to the UK LRNI of 8 mg/day were 24% (95% CI: 22%, 27%). Only 4% reported a dietary iron intake of more than the US recommended intake during pregnancy of 27 mg/day (95% CI: 3, 5%). Mean heme iron intake was 0.6 mg/day (SD = 0.8). This estimate for heme iron changed little after excluding the 114 reported vegetarian participants (with a heme iron intake of zero). Mean non-heme iron intake was 10.9 mg/day (SD = 5.2; Table ).
Average iron intake from food and dietary supplements as reported in first trimester 24-h dietary recall (n= 1257).
In the recall, 20% of participants (95% CI: 18%, 22%) reported taking iron-containing supplements compared with 24% (95% CI: 22%, 26%) in the first trimester questionnaire (Kappa agreement = 0.85). 15% (95% CI: 13%, 18%) and 8% (95% CI: 7%, 10%) reported taking iron-containing supplements in the second and third trimester questionnaires, respectively. Mean total iron intake from diet and supplements, as recorded in the recall, was 16.5 mg/day (SD = 21.1). 34% (95% CI: 32%, 37%) of women had an iron intake >14.8 mg/day from diet and supplements. Only 11 participants reported taking iron-only preparations in the recall, which were assumed to be the conventional therapeutic preparation with a dose of 65 mg iron/tablet, and 5 reported taking a preparation of iron and folic acid that contains 100 mg iron per dose. Mean total iron excluding these 16 participants was 14.3 mg/day (SD = 8.4). Only 8, 21 and 29 participants reported taking iron-only supplements in the first, second and third trimester questionnaires, respectively.
Characteristics of women with high versus low iron intake groups
Women with dietary iron intake >14.8 mg/day were more likely to be older, report a higher total energy intake (Kcal/day), have a university degree, be vegetarian and take daily supplements during the first trimester, including iron-containing supplements. They were less likely to be smokers, live in an area with the worst IMD quartile or have a long-term illness (Table ). Vegetarian participants were less likely to have dietary iron intake ≤14.8 mg/day [unadjusted odds ratio (OR) = 0.5, 95% CI: 0.4, 0.8, P= 0.004]. Vegetarians were also more likely to take iron-containing supplements during the first and second trimester (OR = 2.9, 95% CI: 2.0, 4.3, P< 0.0001 for the first trimester; OR = 2.9, 95% CI: 1.9, 4.4, P< 0.0001 for the second trimester).
Characteristics of women by dietary iron intake during the first trimester reported in a 24-h dietary recall (n= 1257).
There were 1259 babies with information on birthweight. Mean birthweight was 3439 g (SD = 577 g) with 4.4% of babies weighing <2500 g (n= 55). Totally, 13% (n= 166) weighed less than the 10th centile, 8% (n= 99) less than the fifth centile and 5% (n= 65) less than the third centile. Babies weighing more than the 90th centile were 9% (n= 118). Of the 1234 pregnancies with information on gestational age, 55 (4.5%) delivered before 37-week gestation.
Relationship between blood indices and birth outcome
The number of participants who had information on Hb and MCV at 12 and 28-week gestation were 558 and 572, respectively. Mean Hb was 12.7 g/dl (SD = 0.9 g/dl) at 12 weeks and 11.5 g/dl (SD = 1 g/dl) at 28 weeks. The proportion of participants with Hb <11 g/dl was 3% at 12 weeks and 23% at 28 weeks. Mean MCV was 90 fl (SD = 5.0 fl) at 12 weeks and 89 fl (SD = 5.5 fl) at 28 weeks. There was no relationship between customized birth centile or birthweight in grams and Hb/MCV at 12 or 28-week pregnancy in this study. Hb at 28 weeks was associated with SGA (unadjusted OR per g/dl increase in Hb = 1.4, 95% CI: 1.1, 1.8, P= 0.02; OR adjusted for maternal age, salivary cotinine levels and alcohol intake = 1.4, 95% CI: 1, 1.8, P= 0.03). Adjusting for dietary iron intake did not alter this relationship.
Relationship between blood indices and dietary intake
There was no relationship between Hb/MCV at 12 or 28-week pregnancy with dietary iron intake in the first trimester. However, there was a positive relationship between taking iron-containing supplements as reported in the first trimester questionnaire and Hb at 12 and 28 weeks, and MCV at 28 weeks. The relationship remained significant for Hb at 12 and 28 weeks after adjusting for maternal age, ethnicity, parity, educational attainment, vegetarian diet and IMD score in multiple linear regression model. Taking iron-containing supplements in the second trimester was also positively associated with Hb at 28 weeks (Table ).
The relationship between dietary and supplemental iron intake and maternal blood indices (Hb and MCV) during pregnancy.
Relationship between iron intake and birthweight
Dietary iron intake from food was significantly related to birthweight measured on the customized birth centile (unadjusted change per 10 mg/day increase in dietary iron intake during the first trimester = 5.2 centile points, 95% CI: 2.2, 8.2, P= 0.001). Adjusting for maternal age, salivary cotinine levels and alcohol intake attenuated this relationship (adjusted change = 3.1 centile points, 95% CI: −0.2, 6.3, P= 0.07; Table ). The estimate changed little when excluding vegetarians, or including calcium or zinc intake as interaction terms with iron intake (data not shown). Considering birthweight in grams as an outcome, the unadjusted change per 10 mg/day increase in dietary iron intake was 70 g (95% CI: 10, 130, P= 0.02). When adjusting for maternal age, cotinine levels, alcohol intake, maternal weight, height, parity, ethnicity, gestational age and baby's sex, the change was 34 g (95% CI: −13, 80, P= 0.2).
The relationship between maternal dietary iron intake during pregnancy and customized size at birth, Leeds, UK, 2003–2006.
There was no relationship between heme iron intake and customized birth centile (unadjusted change per 1 mg/day increase in heme iron intake = −1.2 centile points, 95% CI: −3.3, 0.8, P= 0.2), while the relationship was statistically significant for non-heme iron (unadjusted change per 1 mg/day increase in non-heme iron intake = 0.6, 95% CI: 0.3, 0.9, P< 0.0001; adjusted change = 0.3, 95% CI: 0, 0.9, P= 0.05). There was a positive relationship between total iron intake, from food and supplements, with customized birth centile (unadjusted change per 10 mg/day increase in total iron intake = 4.3, 95% CI: 2.4, 6.3, P< 0.0001, adjusted change = 2.5, 95% CI 0.4, 4.6, P= 0.02; Table ).
Role of vitamin C intake
The relationship between dietary iron intake from food and customized birth centile was significant in participants with vitamin C intake above 50 mg/day (adjusted change per 10 mg/day increase in dietary iron intake = 3.9, 95% CI: 0.4, 7.5, P= 0.03), compared with −1.9 (95% CI: −11.1, 7.5, P= 0.7, n= 253) for those with vitamin C intake ≤50 mg/day. However, the interaction between iron and vitamin C intakes on the outcome was not significant (P= 0.3). Similar relationships were observed for non-heme iron and total iron intake from diet and supplements using an interaction term between iron intake and vitamin C intake in the models (Table ).
Relationship between iron intake and small for gestational age
Participants with dietary iron intake ≤14.8 mg/day were 1.6 times more likely to have a SGA baby (95% CI: 1.0, 2.5, P= 0.05). However, the adjusted relationship was not significant (1.4, 95% CI: 0.9, 2.3, P= 0.2). This pattern is similar for total iron intake from diet and supplements (Table ).
Relationship between iron intake and preterm birth
There was no relationship between iron intake from diet only, or from diet and supplements, as recorded in the recall diary in the first trimester, and preterm birth (Table ).
Relationship between intake of iron-containing supplements and birth outcomes
There was no association between daily intake of iron-containing supplements in the first and second trimester and customized birth centile. There was an inverse association between taking iron-containing supplements in the third trimester (73% of which as part of multivitamin-mineral preparations) and customized birth centile adjusted for salivary cotinine levels, alcohol intake and maternal age (adjusted difference = −10.7, 95% CI: −16.7, −4.8, P < 0.0001).