During pregnancy, there is a small increase in MCV or mean corpuscular volume, which measures the size of an average red blood cell.
Pregnancy brings about changes in the body that nurture fetal growth and prepare the mother for labor and delivery. Changes may include: hormonal changes, weight gain, and increase in total blood volume including a small increase in MCV.
All these factors affect major organ systems including the muscles, endocrine, heart, lungs, gut, and kidneys. For most women, these changes resolve after delivery with minimal residual effects.
What are the hematological changes in pregnancy?
In a healthy pregnancy, maternal blood volume increases about 50% above normal levels. Your body adapts to sustain the well-being of the fetus, leading to increased plasma volume, red blood cell mass, increased white blood cells, and immunological changes, all of which provide a cushion against expected blood loss at delivery.
Major hematological changes seen during pregnancy include:
- Physiological anemia
- Leukocytosis (increase in white blood cells)
- Immune function changes
- Mild thrombocytopenia (slight reduction in platelet counts)
- Changes in coagulation and fibrinolysis (blood clotting and clot breakdown processes)
Physiologic anemia or dilutional anemia occurs in pregnancy as plasma volume increases more rapidly than red cell mass. The expansion in plasma volume leads to hemodilution, which is a fall in hemoglobin concentration, hematocrit, and red blood cell count.
Symptoms are usually nonspecific, such as fatigue, weakness, light-headedness, and mild dyspnea (breathlessness) during exertion. Sometimes, pallor can develop, and if the anemia is severe, it can cause tachycardia (increased heart rate) or hypotension (low blood pressure). Anemia increases the risk of:
- Preterm delivery
- Low birth weight
- Postpartum maternal infections
The physiologic stress induced by pregnancy is related to leukocytosis, with an increase in neutrophils. The rise in neutrophil count occurs during month 2 of pregnancy and is likely due to impaired neutrophil apoptosis. Leukocyte levels have been associated with cervical dilation and labor progress and resolve to normal levels by day 6 postpartum.
During pregnancy, the mother's immune system undergoes changes to protect both her and her future baby from pathogens (disease-causing agents) while avoiding detrimental immune responses against the fetus.
Pregnancy is associated with increased platelet activation and accelerated clearance. This is partly due to hemodilution, leading to a mean platelet count that is slightly lower than normal. This is called gestational thrombocytopenia and occurs in about 5% of pregnancies. Gestational thrombocytopenia does not lead to complications related to thrombocytopenia, and the babies do not suffer from thrombocytopenia.
Coagulation and fibrinolysis
Pregnancy increases blood clotting tendencies, with higher concentrations of clotting factors such as II, VII, VIII, X, and XII. Pregnancy thus alters the balance of the coagulation system in favor of clotting, predisposing the pregnant and postpartum woman to venous thrombosis (blood clot formation in veins). This risk of thrombosis presents from the first trimester and for at least 12 weeks following delivery.
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Sanghavi M, Rutherford JD. Cardiovascular physiology of pregnancy. Circulation. 2014 Sep 16;130(12):1003-8. https://www.ahajournals.org/doi/full/10.1161/circulationaha.114.009029
Paidas MJ, Hossain N. Hematologic Changes in Pregnancy. In Paidas MJ, Hossain N, Shamsi TS, Rodger MA, Langhoff-RooS J, Lockwood CJ, Eds. Hemostasis and Thrombosis in Obstetrics & Gynecology. 2010. https://onlinelibrary.wiley.com/doi/10.1002/9781444328332.ch1
Bauer KA. Maternal adaptations to pregnancy: Hematologic changes. UpToDate. https://www.uptodate.com/contents/maternal-adaptations-to-pregnancy-hematologic-changes
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