Hematological Profile of Normal Pregnant Women

With the advent of many interventions to improve maternal and child health, pregnant women have become the focus of many health programs. However, few data exist regarding this important population. Although pregnancy-induced changes occur in hematological values, very few laboratories provide specific reference ranges for pregnant women. Most laboratory information systems report reference values based on samples obtained from non–pregnant women which may not be useful for clinical decisions during pregnancy. Thus, there is an increased risk of overlooking important physiologic alterations resulting from pathological conditions and of misinterpreting normal changes as pathological events. It is therefore important to understand pregnancy-induced hematological changes for correct clinical evaluation of pregnant women. In this review, we discuss complete blood count and


Introduction
Pregnancy, also known as gestation or gravidity, is a state in which an embryo(s) implants unto maternal uterus and subsequently develops unto a fetus(es) [1]. Pregnancy starts at conception, when an ovum is fertilized by a spermatozoon to form a zygote, and ends in childbirth, abortion or miscarriage [2,3]. It lasts 40 weeks from last menstruation or 38 weeks from conception date. Normal pregnancy term is 38 to 42 weeks [4][5][6].
With the advent of many interventions to improve maternal and child health, pregnant women have become the focus of many health programs [7][8][9][10]. However, few data exist regarding this important population. Although pregnancy-induced changes occur in hematological values, very few laboratories provide specific reference ranges for pregnant women [11,12]. Most laboratory information systems report reference values based on samples obtained from nonpregnant women which may not be useful for clinical decisions during pregnancy [13][14][15][16]. Thus, there is an increased risk of overlooking important physiologic alterations resulting from pathological conditions and of misinterpreting normal changes as pathological events [17][18][19]. It is therefore important to understand pregnancyinduced hematological changes for correct clinical evaluation of pregnant women [20][21][22].
In this review, we discuss complete blood count and the associated pregnancy-induced hematological changes. We also highlight the dynamic changes of these parameters per trimester and show how they differ between populations (Table 1).

Complete blood count
The complete blood count (CBC) is a blood panel that gives cell type in patient's blood [22,23]. These cell types are red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes) [24]. A normal adult has about 5 L of blood (2 L of blood cells and 3 L plasma). All blood cells are synthesized in the bone marrow's hematopoietic stem cells [25].
Pregnancy increases the proportions of serum, making the RBCs looks fewer in number (pseudo-anemia) [20,36]. With a life expectancy of 120 days, erythrocytes are an excellent long term marker for anemia though it takes long to see the effect of an intervention [35,37]. Erythropoietin production increases with gestational age, peaking at 150% in the third semester. This in turn increases red blood cells by about 33% [31].

Hemoglobin
Hemoglobin (Hgb) is an iron-containing protein complex which binds oxygen and carbon dioxide for transport by RBCs [38]. It forms more than 95% of dry weight of RBCs. Hgb has a quaternary structure with four globular proteins (2α and 2β globulins) and a heme group (fe 2+ which binds oxygen) [31].
In pregnancy, the normal Hgb reference range is 11-12 g/dL. Critical values for Hgb include: Hgb<5 g/dL and Hgb>20 g/dl which can cause heart failure and hemoconcentration-clotting, respectively [13]. Hgb level begins to decline from the 16 th week of gestation as a result of increased plasma volume. Similar trends are seen in RBC count and Hct [12].
Hgb varies with gestational age due to hemodilution and the respective compensatory mechanisms [21]. The high Hgb levels in the first semester are lowered by hemodilution in the second semester while compensatory mechanisms (maternal plasma volume reduction and increased atrial-natriuretic peptides) raise Hgb in the last trimester [39,40]. Similar treads are seen in RBC count and Hct [12]. Anemia in pregnancy is defined by Hgb concentration <11g/dl or Hct<0.33L/L [3]. World Health Organization (WHO) estimates that 51% of pregnant women in developing countries are anemic [20]. Maternal anemia and also high Hbg concentrations leads to preterm birth, perinatal death (abortion), intrauterine growth restriction and low immunity for both mother and child, which exposes them to life threatening infections [3,40]. Low Hgb levels show a poor positive correlation with newborn length but do not show correlation with weight [41]. Women on iron supplementation have stable HB levels throughout the pregnancy [30].

Hematocrit
Hematocrit (Hct), also known as Packed Cell Volume (PCV), is the percentage of RBCs in (centrifuged) whole blood [42]. Normal range is 37-54%. PCV has a wide coefficient of variation hence should be viewed together with other red cell indices [17,43,44]. Elevated Hct (Polycythemia) may reflect erythocytosis. Pregnancy decreases Hct, particularly in the last trimester due to increase in plasma volume. PVC decrease can also be caused by malarial infection, hormonal changes, iron deficiency and conditions promoting fluid retention [9]. Variations in Hct are caused by the same factors as in RBCs and Hgb [14,21]. A Hct <15% and >60% can cause cardiac arrest and spontaneous blood clotting, respectively [29].

Mean cell volume
Mean Cell Volume (MCV) is the average volume of single RBC. It is calculated as Hct/RBC and measured in femtolitres (fL). Microcytic, normocytic, and macrocytic are less than 80, 80-100 and > 100 fL, respectively [31]. Microcytosis (low MCV) is caused by iron deficiency, thalassemia, hemolytic anemia, anemia of chronic disease, hereditary spherocytosis and lead poisoning [30,45]. Macrocytosis (high MCV) is caused by reticuloytosis, folate deficiency, vitamin B 12 deficiency and drugs such as methotrexate and phenytoin [46,47]. In pregnant women, poor positive correlation has been found between maternal Hgb, Hct and MCV. Thus it is important to keep them within the reference range [9].
There is an increase in MCV during pregnancy (an average of 4 fL in an iron-replete woman), which reaches a maximum at 30-35 weeks gestation and does not suggest any deficiency of vitamins B 12 and folate [46,48]. Increased production of RBCs to meet the demands of pregnancy, reasonably explains why there is an increased MCV (due to a higher proportion of young RBCs which are larger in size) [49]. However, MCV does not change significantly during pregnancy and a hemoglobin concentration of 9.5 g/dL in association with a MCV of 84 fL probably indicates co-existent iron deficiency or some other pathology [12].

Mean cell hemoglobin
Mean Cell Hemoglobin (MCH) is the average mass (amount) of Hgb/RBC [50]. Elevated MCH values are found in alcoholism, folate deficiency, vitamin B 12 deficiency, liver disease and hemochromatosis [12,15]. Decreases MCH values are found in sideroblastic anemia, lead poisoning, iron deficiency, anemia of chronic disease and thalassemia [23,37]. There is no significant change in MCH during pregnancy [9].

Platelets
Platelets, also known as thrombocytes, are round to spindleshaped cytoplasmic fragments which contain proenzymes, enzymes, myosin and actin and no nucleus [32,51]. They are produced from megakaryotes which are large bone marrow cells with lobulated nucleus which buds off forming 2-3 nm platelets [19]. Platelet average number per µL is 350,000 (range: 150,000-500,000). Thrombocytes contribute to hemostasis by clumping together and sticking to vessel walls (platelet phase) which activate intrinsic pathway of coagulation phase [23,24]. Platelets have a lifespan of 7-12 days [51].
There is a significant decrease in platelets count with gestational age [2]. During pregnancy, the uterine wall continuously expands to accommodate fetal growth. This causes laceration of blood vessels at the uterus leading to massive hemorrhage. The primary hemostatic plug where these tears occur is formed by platelets [12,13]. Thrombocytopenia occurs in 8% of all pregnancies, and is the second common hematological disorder after anemia [20]. Thrombocytopenia is defined by platelet count <150*10 9 /L. Gestational thrombocytopenia is due to increased platelet destruction by their activation and increased clearance [12]. Hemodilution also contributes to gestational platelet reduction. The immune system responds by synthesizing newer and larger platelets. Levels of clotting factors and fibrinogen also increase while fibrinolytic activity decreases. These changes are meant to protect the expectant mother from hemorrhage at delivery [11]. However, the increased platelet aggregation, especially in the third trimester makes, makes gestation a hypercoagulable state prone to thromboembolism [2,3]. Gestational thrombocytopenia needs no medical intervention and remises after delivery. However other causes such as immune thrombocytopenia, megaloblastic anemia, liver disorders thrombotic microangiopathy syndromes and eclampsia must be excluded [27,41].
Women with twins may have lower platelet count compared with singleton pregnancies, probably due to higher thrombin generation. Thrombocytopenia in pregnancy is generally mild, with only few life threatening cases [2].

Mean platelet volume
Mean Platelet Volume (MPV) is the calculated measurement of mean platelet size. Platelet size often increases with increased production [42,50]. Elevated MPV is diagnostic of immune thrombocytopenic purpura (ITP), myeloprolifera disease, Bernard-Soulier syndrome or pre-eclampsia [31,41]. Reduced MPV is diagnostic of aplastic anemia, cytotoxic drug therapy or viral infections [36].

White blood cells
White blood cells (WBC) are classified into agranulocytes (lymphocytes and monocytes) and granulocytes (eosinophils neutrophils, and basophils) [5]. Granulocytes, also referred to as polymorphonuclear cells, are the most abundant and highly motile [26]. Leukocytes play a large role in the acute phase of inflammation (especially bacterial infections) through phagocytosis. They also supply antibodies [11,12]. WBC are synthesized in the bone marrow and destroyed after 14 to 21 days in the lymphatic system [53]. Leukocytes average number per µL is 7000 (range: 5000-10,000). A patient with leucocytes <500 risks fatal infection while a patient with leucocytes >30,000 has serious disease like leukemia) or massive infection [18,37].
Normal pregnancy is accompanied by leukocytosis, caused by physiological stress. A complex physiological process increases leucocyte counts during pregnancy. Serial changes occur in endocrine system, metabolic processes and genital system. Therefore, leucocytes increase because they accept stimulatory signals as pregnancy progresses [10,11]. Studies have shown that gestational leukocytosis is as a result of leucocytes release from marginal pools [2,41]. Leukocytosis begins in the first trimester and remains high throughout pregnancy [21]. The white blood cell count during normal pregnancy is between 6*10 9 and 16*10 9 /L [3]. The WBC count normalizes 4 weeks after delivery [7,12]. The leukocytosis-inducing physiological stress is attributed to elevated inflammatory response due to immunomodulation, immunosuppression and selective immune tolerance of fetus. However, Yu et al. [2016] suggests that leukocytosis occurs after normal delivery [9,52]. While Tzur et al. [54] associates leukocytosis in first trimester with complicated pregnancy. Thus correct interpretation and correlation of leukocyte count is important during antenatal care and postnatal [20]. Amongst leucocytes, preponderance of neutrophils on differential counts during pregnancy is due to impaired neutrophilic apoptosis [34]. The neutrophil count can be twice its postpartum values [45]. Bone marrow hyperplasia during the last trimester is associated with neutrophilic leukocytosis [21]. There is a significant increase in white blood cell and neutrophil count on day one postpartum which then decreases and normalizes by fifth day. This should be considered to avoid unnecessary use of antibiotics. WBC count peak at delivery hence is a limited marker for infection during normal birth. The increase in leukocyte count is mainly from increased neutrophil and lymphocyte counts [12,41].

Neutrophils
Neutrophils, also known as polymorphonuclear leukocytes, are the main phagocytic leucocytes which engulf and digest pathogens or debris in tissues and release cytotoxic enzymes and chemicals (such as bactericides, prostaglandins and leukotrienes) [17,55]. Neutrophil average number per µL is 4150 (range 1800-7300) with a differential count of 50-70%. They are round cells with 3 to 5-lobed nucleus with large, pale inclusions in the cytoplasm [13,56]. They move into tissues after a few hours and may survive for days depending on tissue activity. They are produced in red bone marrow. Etiologies for neutrophilia (high neutrophil count) are: acute bacterial infection, acute stress, burns, leukemia, steroids, rheumatoid arthritis [23,46,[57][58][59]. Etiologies for neutropenia (low neutrophil count) are: folate/ vitamin B 12 deficiency, aplastic anemia, chemotherapy and medications such as chloramphenicol and sulfonamides [17,22,25]. Neutrophils increases during pregnancy [3]. Neutrophilia in pregnancy is probably as a result of impaired neutrophilic apoptosis. Pregnant woman serum has inhibitory factors which depress neutrophil chemotaxis and their phagocytic activity [11].

Lymphocytes
Lymphocytes are round cells, slightly larger than RBCs, with round nucleus and very little cytoplasm. They are the defensive cells of lymphatic system. They can survive for decades, circulating between blood and tissues [15,31]. There are three main types of lymphocytes; T cells, B cells and Natural Killer (NK) cells. T cells mature in the thymus. There are multiple T cells, the most common are: Helper T cells which help other cells mature, activate and function through cytokine production; cytotoxic T cells which cells destroy infected with viruses, and memory T cells which remembers the antigens of past infections [27,43]. B cells are formed in bone marrow. There are two types of B cells: plasma B cells which produce antibodies and memory B cells which remember past infections. NK cells kill infected or cancer cells [22,51]. Lymphocytes average number per µL is 2185 (range: 1500-4000); differential count: 20-30% [25]. Lymphocytosis (high lymphocyte count) is caused by viral infections, leukemias and adrenal insufficiency. Lyphocytopenia (low lymphocyte count) is caused by; HIV virus which destroys T cells (CD4), aplastic anemia, glucocorticoid, rheumatoid arthritis among others [43,56].
A review by Okpokam et al. [39] found that lymphocytes increases during pregnancy probably due to bacterial infections. However, Elado et al. [3] observed lymphocyte suppression with gestational age.