Sara's hemoglobin at her initial prenatal visit was 12.5 g/dl. Her repeat hemoglobin at 27 weeks is 11.7 g/dl. Which of the following factors explains this finding?
1. Physiological hemodilution due to increased plasma volume during pregnancy.
2. Iron deficiency anemia.
3. Dietary changes affecting hemoglobin synthesis.
4. Normal fluctuations in hemoglobin levels during pregnancy.
