2.3.2. Blood Sampling and Biomarkers

Fasting blood samples were collected from participants at 34–39 weeks of gestation. Blood samples were collected in serum separation tubes (VENOJECT2; Terumo, Tokyo, Japan). After collection, the serum was centrifuged at 3500 rpm for 5 min at −80 °C, and then stored in a −80 °C freezer. The samples were evaluated within 17 months. Blood samples were used to measure ferritin levels. The biomarker measurements were performed in a contracted laboratory (SRL Inc., Tokyo, Japan). Ferritin levels were measured using the chemiluminescent enzyme immunoassay method.

Biomarker data, which included Hb, Hct, MCV, mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC) and red cell distribution width (RDW), were obtained from the patients’ medical records. Hb levels in the first trimester (T1) were measured for up to 13 weeks, and Hb levels in the second trimester (T2) were measured between 18 weeks and 27 weeks. Hb, Hct, MCV, MCH, MCHC, and RDW in T3 were collected close to 35 weeks of gestation.