NORMAL CHANGES DURING PREGNANCY

During pregnancy, the body undergoes normal physiologic changes in the cardiovascular, hematologic, renal, and gastrointestinal systems to accommodate and support the developing fetus. In the absence of illness, the body can generally compensate for these changes. However, in the presence of conditions such as anemia, clotting disorders, increased bleeding, preeclampsia, and trauma caused by motor vehicle accidents or intimate partner violence (IPV), the body may not be able to compensate for the changes.

When caring for pregnant patients, nurses must be aware of both the normal and abnormal physiologic changes that occur during pregnancy and the resulting laboratory values. This enables them to identify situations early and plan appropriate interventions.

Cardiovascular and Hematologic Changes

Maternal circulation changes during pregnancy to accommodate an increase in blood volume of up to 50%, including a 45% to 50% increase in plasma volume and 20% to 30% increase in red blood cells. Since these percentages are not equal, the subsequent hemoglobin/hematocrit will reflect a normal physiologic anemia of pregnancy.

During pregnancy, the systemic vascular resistance (SVR) of the blood vessels lowers due to increased levels of hormones. This decreasing SVR is an expected result of the increasing progesterone and prostaglandin levels, which relax smooth muscle, producing vasodilatation.

As a result of the increased volume and decreased resistance, cardiac output (and heart rate) rise. Therefore, there is a normal lowering of the blood pressure, especially in the second trimester. This sometimes causes dizziness or feeling faint in women as they rise to standing during the second trimester. Their pressure should stabilize and approach prepregnancy numbers by the third trimester.

White blood cell (WBC) counts, especially neutrophils, increase naturally during pregnancy. During active labor there may be another normal increase, even in the absence of infection. In nonpregnant patients, a normal WBC count is somewhere between 5 and 10 (5,000–10,000 cells/mm³), but for pregnancy, those normal values can be between 6 and 16 in the third trimester and may reach 20 to 30 in labor and early postpartum. When evaluating for infection, therefore, you need to look for other clinical indicators—such as increased temperature, bacteriuria, WBC in urine, uterine tenderness, and fetal tachycardia—and document them.

NORMAL HEMATOLOGIC VALUES

	Nonpregnant	Pregnant – 1st trimester	Pregnant – 2nd trimester	Pregnant – 3rd trimester
(Cunningham, 2022)
Hemoglobin (HGB) (g/dL)	12–15.8	11.6–13.9	9.7–14.8	9.5–15
Hematocrit (HCT)	35.4%–44.4%	31.0%–41.0%	30.0%–39.0%	28%–40%
Red blood cells (RBC) (x106/mm3)	4.0–5.2	3.42–4.55	2.81–4.49	2.71–4.43
White blood cells (WBC) (x106/mm3)	3.5–9.1	5.7–13.6	5.6–14.8	5.9–16.9

Pregnancy is typically considered a hypercoagulable state—meaning that most pregnant women clot more readily than normal and are predisposed to deep-vein thrombosis or other clot-related conditions. During pregnancy there is an increase in certain factors in the clotting cascade due to normal adaptation. Platelets are usually unchanged in pregnancy, and increased levels of platelets are rare.

Renal Changes

The renal system undergoes many changes in pregnancy to accommodate increased metabolic and circulatory requirements. The system clears the body of both maternal and fetal waste and is affected by the increased blood volume and lowered systemic vascular resistance. As previously mentioned, progesterone has a relaxing effect on vascular tissue, thus enhancing the renal blood flow and function. The increased plasma flow into the renal system causes the glomerular filtration rate (GFR) to rise dramatically.

Renal clearance of many substances is generally elevated in pregnancy, causing lower-than-usual serum levels of the renal markers blood-urea-nitrogen (BUN) and creatinine. Increased filtration does not mean enhanced reabsorption, however. The increase in glucose load during pregnancy is often spilled into the urine and not reabsorbed. Therefore, spillage of some glucose in pregnancy is not always indicative of pathology.

The anatomy of the pregnant uterus causes changes in the location and pressure of other internal organs. The bladder becomes slightly concave and is displaced forward and upward. The uterus causes the ureters to become dilated and more tortuous, especially the right ureter. Detectable hydronephrosis or hydroureter (i.e., dilation of the renal pelvis and ureter caused by obstruction to free flow of urine) during pregnancy is considered normal and may take 3–4 months post delivery to fully resolve.

NORMAL VALUES FOR RENAL FUNCTION

	Nonpregnant	Pregnant
(Cunningham, 2022)
Serum creatinine	0.5–0.9 mg/dL	0.53–0.9 mg/dL decrease
Serum BUN	7–20 mg/dL	8–10 mg/dL decrease
Serum uric acid	2.5–5.6 mg/dL	2–6.3 mg/dL
Urine creatinine clearance	91–130 mL/min	69–166 mL/min
Urine uric acid	150–990 mg/24 hrs	Increases
Urine glucose	60–115 mg/dL	Increases

Gastrointestinal Changes

Increased progesterone levels also affect the gastrointestinal (GI) system of the pregnant woman. General tone, lower esophageal sphincter tone, and motility are decreased. This predisposes the woman to increased incidence of reflux (heartburn) and constipation. As the gravid uterus displaces the internal organs, this incidence increases.

The liver increases its production of lipids and cholesterol. This change, combined with delayed gallbladder contraction (due to progesterone-influenced relaxation), may lead to increased gallstone formation (expected) or inflammation of the gallbladder (abnormal). The liver also plays a role in the production of the clotting factors.

In addition to increased production of lipids and certain clotting factors, some enzymes found within the liver are also increased without indicating pathology. It is important to distinguish a normal rise in these levels from a pathologic change caused by organ damage or destruction arising, for example, from preeclampsia or hepatitis. In preeclampsia, microclots in the liver and capsular edema are danger signs, and if clotting factors become affected, the patient is at a high risk for disseminated intravascular coagulation (DIC). Diagnoses are not based upon a single abnormal value.

NORMAL HEPATIC VALUES

Liver Enzymes	Nonpregnant	Pregnant
(Cunningham, 2022)
Alanine transaminase (ALT)	4–71 units/L	Unchanged
Aspartate aminotransferase (AST)	12–38 units/L	Unchanged
Alkaline phosphatase (ALP)	33–96 units/L	> up to 2–4 times
Lactate dehydrogenase (LDH)	115–221 units/L	Upper end of normal to 524 units/L

Abnormal hepatic lab values can indicate such conditions as cholelithiasis, preeclampsia, hepatitis, cholestasis, and HELLP syndrome.