Normal Pregnancy

Physiological Changes During Pregnancy

Hematologic System

Along with changes to the endocrine system during pregnancy, some changes occur to other systems within the pregnant individual. For instance, there are several changes to the hematologic system. These changes include a 50% increase in the volume of plasma, a 30% increase in the volume of red blood cells (RBC’s), and an increase in the amount of white blood cells (WBC’s). These changes to the hematologic system serve several purposes. The overall increase in the volume of blood circulating is in preparation for the expected loss of blood during the birthing process. However, the 50% increase in plasma versus the 30% increase in RBC’s results in the level of hemoglobin being 10.5 to 11.5 g/dl, resulting in dilutional anemia.[1]

Another change to the hematologic system includes an increase in factors that help with coagulation. A fibrin degradation inhibitor is also synthesized by the placenta. All of these changes to the hematologic system contribute to pregnancy being in a state of hypercoagulability, making pregnant women more prone to thromboembolic disease.

Cardiovascular System

Pregnant individuals also have altered cardiovascular systems. Increases in stroke volume and pulse can lead to an overall increase in cardiac output. Conversely, peripheral vascular resistance is decreased, and in the second trimester there is often a decrease in blood pressure. Approximately 9 out of 10 pregnant individuals have an S3 gallop and systolic ejection murmur.

Respiratory System

The respiratory system also undergoes several changes during pregnancy. The increase in estrogen causes the mucosa of the nasal cavity to swell with excess mucous and fluid. The respiratory rate and tidal volume are greater than normal, and the residual volume is less than normal. Toward the later stages of pregnancy, it is normal for a woman to experience some difficulty breathing. One of the possible causes of dyspnea is the enlarged uterus pressing up against the lungs, reducing the volume of oxygen that can be exchanged in one breath.[2]

Renal System

Several changes to the renal system occur during pregnancy as well. For instance, the glomerular filtration rate (GFR) and renal plasma flow (RPF) rise by 50 and 75 percent, respectively. The amount of plasma filtered of creatinine per unit time also rises. The kidneys themselves also get larger and, in turn, weigh more. The ureters, which connect the kidneys to the bladder, also enlarge, making their central cavity wider than normal. The bladder, which usually resides in the pelvis, becomes situated in the abdomen. Due to all these changes in the renal system, it is more common for pregnant people to have to urinate more often and sometimes not have total control over urination. Pregnant individuals are also more prone to kidney infection, and, if they are involved in a traumatic incident affecting the abdomen, a ruptured bladder will often be observed.

Gastrointestinal (GI) System

During pregnancy, the levels of progesterone are increased, impacting the movement of waste through the GI system. This can lead to constipation. Increased progesterone can also cause nausea and vomiting during the early stages of pregnancy, and in more severe cases may lead to hyperemesis gravidarum. It is also common for pregnant women to feel as if they are not hungry even though they have not eaten for a while. Due to the stomach and esophagus being pushed upon and moved by the enlargement of the uterus, many pregnant people will also experience heartburn during pregnancy.

Integumentary System

Noticeable changes to the integumentary system, especially around the abdominal area, occur during pregnancy. As the uterus grows with the developing fetus, stress is put on the surrounding skin, often resulting in stretch marks. This is due to the collagen in the skin parting and can result in an itchy feeling in the affected area. The last 20 weeks of pregnancy are when women will start to see striae gravidarum on their stomach, which post-pregnancy will become striae albicans (stretch marks).

Reproductive System

The last major system that changes during pregnancy is the reproductive system. The uterus goes from 70 gm to 1100 gm, often causing lower back and leg pain. The amount of blood supplying the uterus also increases from around 2-3% to approximately 10-15% during pregnancy. The cervix and vagina also change during pregnancy. They receive more blood flow, which can be seen via Chadwick’s sign, a bluish discoloration in the area. There is also an increase in the amount of mucus secreted by the cervix. Due to these changes, the vagina is often more sensitive, which is noticeable during sexual intercourse while pregnant. The breasts also change during pregnancy as they get larger, more sensitive, and often somewhat lumpy. The nipple and areola will also become noticeably darker in color.

  1. Gandhi MH, Gupta V. Physiology, Maternal Blood. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023.
  2. McCormack, Meredith C., and Robert A. Wise. “Respiratory Physiology in Pregnancy.” Humana Press, 2009, pp. 19–26.


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Human Reproduction: A Clinical Approach Copyright © 2023 by Dr. Hala Bastawros, M.D is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.