This study aims to explore how body fat influences the response to either 25 or 50 mg of daily iron supplements during pregnancy. We will conduct a double-blind randomized controlled intervention study involving 312 pregnant women recruited from antenatal clinics in the Northern Trust Area. Participants will be randomly assigned to receive either 25 or 50 mg of iron per day from 12 weeks of pregnancy until delivery, using the Active Iron supplement brand. Blood samples will be collected at 12, 28 and 36 weeks gestation and umbilical cord blood will be collected at delivery. Anthropometric measurements will be taken at each visit, and participants will complete questionnaires on various aspects of health and lifestyle, mental health, gastrointestinal symptoms, and compliance.

The main aim of this study is to investigate the influence of adiposity on the difference in response to 25 or 50 mg of daily iron supplementation during pregnancy. The primary aim is to determine the influence of maternal adiposity on adjusted maternal ferritin concentrations in response to 25 mg or 50 mg iron supplementation in pregnancy. The secondary outcomes of this study include: investigating the impact of maternal body fat on various maternal iron biomarkers (such as haemoglobin, soluble transferrin receptor, hepcidin, transferrin saturation, and other haematological markers) in response to either 25 mg or 50 mg iron supplementation during pregnancy, evaluating changes in adjusted ferritin concentrations and other iron markers throughout pregnancy relative to the dosage of iron supplementation received, determining the effect of maternal body fat on neonatal iron biomarkers in response to maternal iron supplementation, assessing changes in markers of inflammation in response to iron supplementation during pregnancy, and examine changes in mental health scores in response to iron supplementation during pregnancy.

This is a double-blind randomised controlled intervention study, in which 312 pregnant women with singleton pregnancy, without current complications, aged ≥ 18 years and BMI ≥ 18.5 kg/m2 will be recruited. Participants who are taking multivitamins will be included. They will be asked to discontinue any current supplementation. Pregnant women with anaemia, iron deficiency, high risk of iron overload, history of bariatric surgery, who are planning home birth, are currently involved in another research study, and those who cannot speak or understand English language will be excluded.

Blood samples and anthropometric and body composition measurements will be taken at different points in the pregnancy (12, 28, 36 gestational weeks) and an umbilical cord blood sample at the time of birth. Blood concentrations of iron and inflammation markers will be analysed. General, dietary intake and lifestyle information will be collected, through a Health and Lifestyle questionnaire and a 4-day diary. Additionally, participants will complete a questionnaire about their mental health and gastrointestinal symptoms. The compliance of the supplementation will be evaluated at each timepoint. Additionally, participants will receive a telephone call to evaluate possible adverse effects and compliance of the supplementation between the timepoints (18, 24 and 32 weeks of gestation). In the event that a participant has been prescribed iron treatment, anaemia diagnosis at any time during pregnancy or the occurrence of any adverse outcome such as miscarriage, the participant will be withdrawn from the study. Electronic forms prepared in RedCap will be used to collect data.

Amendment to above protocol:

Removal of one of the exclusion criteria of the study: having a ferritin concentration < 30 ug/L at time of enrolment. Currently, ferritin is not part of routine investigations at any time during pregnancy, unless there are risk factors for iron deficiency (Pavord et al 2019).

Excluding pregnant women with low iron levels will select a very low risk population of developing iron deficiency, and therefore, they will not reflect the real effect of the intervention in this population.

2) Include an 8 week postpartum sampling timepoint. The intervention will conclude as originally planned during the delivery and will not be extended. An additional timepoint will be offered and explained to participants who are already enrolled, but they will not be obligated to participate.

Postpartum anemia affects 25-30% of women (Bodnar et al., 2005). Additionally, recent unpublished data from Northern Ireland shows that 80% of women receiving 17 mg/day of iron supplementation throughout pregnancy were iron deficient by 36 weeks, with even higher rates among pregnant women with obesity. Given these findings, we will extend our assessment of iron status after the intervention by adding this postpartum timepoint.

Inclusion Criteria:- Pregnant women

• Age ≥ 18 years
• BMI ≥18.5 kg/m2
• Without current pregnancy complications (for example, severe bleeding, Diabetes Mellitus, hyperemesis gravidarum, ectopic and molar pregnancies)
• At least 12 Gestational Week
• Singleton pregnancy confirmed with the first ultrasound scan
• Participants who are currently taking multivitamins will be included. They will be asked to discontinue any current supplementation.

Exclusion Criteria:

• Hb <110 g/L
• SF <30 μg/L
• High risk of iron overload (Hb >150 g/L, transferrin saturation >45% or SF> 150 μg/L)
• Participants with history of haematological, renal, liver, autoimmune disorders, malabsorptive syndromes
• Participants with history of bariatric surgery
• Participants who take steroids or anti-inflammatory treatments or drugs that affect gut absorption (proton-pump inhibitors)
• Planned home births
• Participants currently involved in another research study
• Multiple pregnancy
• Participants who do not speak English

only female participants are being studied

Double Blind: two or more parties are unaware of the intervention assignment