Physiological Changes in Pregnancy

Physiological changes occur in pregnancy to nurture the developing fetus and prepare the mother for labor and delivery. Some of these changes influence normal biochemical values while others may mimic symptoms of medical disease. It is important to differentiate between normal physiological changes and disease pathology.

What are Physiological changes?

Pregnancy causes physiologic changes in all maternal organ systems; most return to normal after delivery. These changes happen in response to many factors; hormonal changes, increase in the total blood volume, weight gain, and increase in fetus size as the pregnancy progresses. All these factors have a physiological impact on the pregnant woman: the musculoskeletal, endocrine, reproductive, cardiovascular, respiratory, nervous, urinary, gastrointestinal, and immune systems are affected, along with changes to the skin and breasts. The full gestation period is 39-40 weeks, and pre-term birth is classed as delivery before 37 weeks gestation, although there is variation internationally and it is thought that the length of human pregnancies also varies naturally.

The primary function of pregnancy is to allow for the growth and development of the fetus. All changes that occur within the mother’s body are intended to allow for this growth, as well as for the development of the placenta to nourish the fetus and sustain the pregnancy. 

A pregnant woman will also become hypercoagulable, leading to an increased risk of developing blood clots and embolisms, such as deep vein thrombosis and pulmonary embolism. Women are 4-5 times more likely to develop a clot during pregnancy and in the postpartum period than when they are not pregnant. Hypercoagulability in pregnancy likely evolved to protect women from hemorrhage at the time of miscarriage or childbirth. In third-world countries, the leading cause of maternal death is still hemorrhage. In the United States 2011-2013, hemorrhage made up 11.4% and pulmonary embolisms made up 9.2% of all pregnancy-related deaths.

The increased risk of clots can be attributed to several things. Plasma levels of pro-coagulation factors increased markedly in pregnancy, including von Willebrand Factor, fibrinogen, factor VII, factor VIII, and factor X. Both the production of prostacyclin (an inhibitor of platelet aggregation) and thromboxane (an inducer of platelet aggregation and a vasoconstrictor) are increased, but overall there is an increase in platelet reactivity which can lead to a predisposition to clots. There is also increased blood stasis due to the compression of the vena cava by the enlargening uterus. Many factors have been shown to increase the risk of clots in pregnancy, including baseline thrombophilia, cesarean section, preeclampsia, etc. Clots usually develop in the left leg or the left iliac/ femoral venous system. Recently, there have been several case reports of May-Thurner Syndrome in pregnancy, where the right common iliac artery compresses the below left common iliac vein.

In general, the changes are more dramatic in multifetal than in single pregnancies. These physiologic changes allow the parturient to support the growing uterus and fetus and to withstand labor and the postpartum course. Because the gravida may require surgery while pregnant, it is important to have an appreciation of these physiologic changes. 

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