What causes low hemoglobin in pregnancy?

Causes of Low Hemoglobin in Pregnancy

The primary cause of low hemoglobin (anemia) in pregnancy is iron deficiency, accounting for approximately 75% of all cases, due to increased iron requirements during pregnancy and insufficient iron stores to meet these demands. 1

Physiological Changes Contributing to Low Hemoglobin

• Hemodilution: Normal physiologic changes in pregnancy lead to plasma volume expansion that outpaces the increase in red blood cell mass, causing a relative reduction in hemoglobin concentration 2
• Increased iron requirements: Pregnancy demands additional iron for:
  • Fetal development
  • Placental growth
  • Expansion of maternal erythrocyte mass
  • Preparation for blood loss during delivery 3

Common Causes of Low Hemoglobin in Pregnancy

Iron Deficiency (Primary Cause)

• Insufficient dietary iron intake
• Poor iron absorption
• Pre-pregnancy iron deficiency
• Closely spaced pregnancies without adequate iron repletion
• Multiple gestation pregnancies (increased demand) 2

Other Nutritional Deficiencies

• Folate deficiency (second most common cause of anemia in pregnancy)
• Vitamin B12 deficiency (particularly in vegetarian/vegan diets) 4

Hemoglobinopathies

• Thalassemia (particularly in women of Mediterranean, African, or Southeast Asian ancestry)
• Sickle cell disease or trait 5

Blood Loss

• Acute bleeding (placental issues, gastrointestinal bleeding)
• Chronic blood loss (heavy menstrual periods before pregnancy) 1

Timing and Severity

• First trimester: Hemoglobin <11.0 g/dL is considered anemia
• Second/third trimester: Hemoglobin <10.5 g/dL is considered anemia 3
• Prevalence increases throughout pregnancy:
  • First trimester: As low as 1.8%
  • Third trimester: Up to 27.4% 5

Risk Factors for Iron Deficiency Anemia

• Poor nutrition or diets low in iron
• Multiple pregnancies
• Closely spaced pregnancies
• Adolescent pregnancy
• High-altitude residence
• Smoking (requires adjustment of hemoglobin cutoffs) 5, 2

Clinical Implications

• Mild to moderate iron deficiency may not significantly affect fetal hemoglobin concentration
• Severe anemia (Hb <6 g/dL) is associated with:
  • Preterm birth (35% increased risk)
  • Low birth weight (28% increased risk)
  • Small-for-gestational age infants
  • Stillbirth (43% increased risk)
  • Perinatal mortality
  • Postpartum hemorrhage (69% increased risk)
  • Increased need for blood transfusion 1, 6

Prevention and Management

• Routine screening at first prenatal visit and at 24-28 weeks gestation 3
• Prophylactic low-dose iron supplementation (30 mg/day) starting at first prenatal visit 5, 2
• For diagnosed anemia, increase to 60-120 mg/day of elemental iron 5, 2
• Encourage consumption of iron-rich foods and vitamin C to enhance absorption 2
• Consider IV iron for those who cannot tolerate oral iron or have severe anemia 3

Important Pitfalls to Avoid

• Failure to distinguish between physiologic anemia of pregnancy and true iron deficiency
• Overlooking non-iron deficiency causes in non-responsive cases
• Not adjusting hemoglobin cutoffs for altitude or smoking status
• Assuming high hemoglobin is beneficial (values >15.0 g/dL in second/third trimester may indicate poor blood volume expansion and increased risk of adverse outcomes) 5
• Neglecting to continue iron supplementation postpartum in women with anemia or significant blood loss during delivery 2

Proper diagnosis and management of low hemoglobin in pregnancy is essential to prevent adverse maternal and fetal outcomes, with early intervention being key to successful treatment.