Hemoglobin (Hgb) is a complex protein within red blood cells responsible for transporting oxygen throughout the body. During a typical pregnancy, the total volume of blood increases significantly, but the liquid component (plasma) increases more than the red blood cell mass. This disproportionate increase leads to blood dilution, meaning a mild drop in hemoglobin (physiologic anemia of pregnancy) is expected. An elevated hemoglobin level is an unexpected finding that warrants attention, suggesting the blood may be too concentrated and potentially signaling a problem for both the mother and the baby.
Understanding Elevated Hemoglobin Levels
An elevated hemoglobin level is often referred to as polycythemia or erythrocytosis, indicating a higher-than-normal concentration of red blood cells. Given the expected blood dilution, a hemoglobin level above 13.0 g/dL in the second or third trimester is often considered high and prompts further investigation. It is crucial to distinguish between two categories: absolute and relative polycythemia. Absolute polycythemia involves an actual increase in the total number of red blood cells, which can be due to primary or secondary causes. In contrast, relative polycythemia, also known as hemoconcentration, is an apparent elevation. This occurs when the red blood cell mass is normal, but the plasma volume is reduced, concentrating the blood components. Relative polycythemia is the more common finding in pregnancy.
Primary Reasons for High Hemoglobin in Pregnancy
The most frequent cause of elevated hemoglobin during pregnancy is hemoconcentration due to dehydration. When a pregnant person does not consume enough fluids or experiences excessive fluid loss (such as from severe vomiting), the plasma volume temporarily shrinks. This reduction in the liquid component concentrates the blood, leading to a high reading on the lab test.
A high hemoglobin level can also be a bodily response to chronic low oxygen levels, known as hypoxia, leading to secondary absolute polycythemia. This condition causes the body to produce more red blood cells in an attempt to carry more oxygen. Maternal factors that can cause chronic hypoxia include smoking, living at high altitudes, or chronic respiratory diseases, such as severe chronic obstructive pulmonary disease (COPD).
Rarely, an elevated hemoglobin level can be a sign of a primary blood disorder, such as Polycythemia Vera (PV). PV is a type of myeloproliferative neoplasm where the bone marrow produces too many red blood cells. Although rare in reproductive-aged women, this condition represents a true increase in red blood cell mass and requires careful management in pregnancy. Identifying the root cause is necessary because the treatment approach will vary significantly.
Potential Risks for Mother and Fetus
The primary concern associated with elevated hemoglobin, especially in absolute polycythemia, is the resulting increase in blood viscosity, meaning the blood becomes thicker. This hyperviscosity slows blood flow, which significantly raises the mother’s risk of developing thromboembolic events, such as deep vein thrombosis (DVT) or pulmonary embolism (PE). Blood clots pose a serious danger and require prompt intervention.
For the developing fetus, elevated maternal hemoglobin is often linked to complications resulting from impaired blood flow to the placenta. High maternal blood viscosity can impede the uteroplacental circulation, reducing the delivery of oxygen and nutrients to the baby. This reduced efficiency can lead to poor fetal outcomes, including the baby being born small for gestational age (SGA) or experiencing intrauterine growth restriction (IUGR).
Furthermore, an elevated hemoglobin level, particularly in the first and second trimesters, has been linked to an increased risk of developing preeclampsia or gestational hypertension. Preeclampsia is a serious condition characterized by high blood pressure and signs of organ damage. This condition can be exacerbated by the relative lack of plasma volume expansion that often accompanies hemoconcentration. The finding is an indicator of a potential physiological challenge to the pregnancy.
Clinical Management and Monitoring
Once a high hemoglobin level is detected, clinical management involves diagnostic evaluation to determine the underlying cause. Healthcare providers first attempt to rule out relative polycythemia by assessing the patient’s hydration status, especially if they have experienced prolonged nausea, vomiting, or insufficient fluid intake. If dehydration is suspected, aggressive rehydration is the initial intervention, which often corrects the hemoglobin concentration quickly.
If the high level persists after hydration, the focus shifts to identifying secondary causes, such as chronic smoking or underlying cardiopulmonary disease, or the rare primary condition like Polycythemia Vera. Management for confirmed Polycythemia Vera typically involves serial phlebotomies to reduce blood viscosity and the risk of clots. Daily low-dose aspirin therapy is also commonly prescribed to mitigate the thrombotic risk.
Throughout the remainder of the pregnancy, specialized surveillance is required, particularly if the high hemoglobin level is associated with a chronic condition. Maternal monitoring includes regular blood pressure checks and screening for signs of thromboembolism. Fetal surveillance involves serial ultrasounds to assess fetal growth and amniotic fluid volume, along with Doppler flow studies to evaluate blood flow in the umbilical and uterine arteries, ensuring adequate uteroplacental circulation.