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Pregnancy and Hypotension (Low Blood Pressure)

Note: The Pregistry website includes expert reports on more than 2000 medications, 300 diseases, and 150 common exposures during pregnancy and lactation. For the topic Hypotension, go here. These expert reports are free of charge and can be saved and shared.
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In a previous post, a part of our pregnancy physiology series, we discussed the circulatory system and how it changes during pregnancy. We learned that changes in this body system are rather dramatic as pregnancy advances. The volume of blood within your body increases by up to 50 percent by the end of pregnancy. This causes decreasing concentration in red blood cells (RBCs), causing a drop in the RBC count, along with a drop in hemoglobin. The body begins compensating for the dilution of RBCs by increasing RBC production. Meanwhile, the expansion of the blood volume compensates for your growing uterus, which requires a great deal of blood flow, for blood loss during delivery, and for expansion of blood vessels throughout the body to allow for blood to flow with decreased resistance. This is called a reduction in the systemic vascular resistance. In some women, this together with hormonal changes and other factors can lead the blood pressure to fall lower than normal. Hypotension, low blood pressure, is thus very common among pregnant women, but its prevalence varies widely between different reports. The chances that you will suffer an episode of hypotension rise substantially if you have a cesarean delivery with spinal anesthesia.

Throughout pregnancy, your blood pressure is taken with a blood pressure cuff, which measures systolic pressure (pressure at the peak of ventricular contraction) and diastolic pressure (lowest pressure during the period between ventricular contractions) in your arm. Hypotension is diagnosed if systolic pressure is below 90 mm Hg (some experts say 100mmHg) or diastolic pressure is below 60 mm Hg, which is to say that you are hypotensive if your blood pressure is less than 90/60 mmHg. Your blood pressure also may be measured lying down, sitting, and standing to see if you have what doctors call orthostatic hypotension, which can result from certain medications. Orthostatic hypotension is a condition in which the blood pressure drops a lot when you stand up.

Hypotension can make you feel dizzy, lightheaded, nauseous, tired, and confused. It reduces your ability to concentrate, blurs your vision, and can cause you to faint. If severe, hypotension can lead to shock, a potentially fatal condition that is characterized by pale, cold, clammy, skin, a rapid, weak pulse, and rapid, shallow breathing. A hypotensive pregnant woman also may have inadequate blood flow through the uterus and placenta. This can cause intrauterine growth retardation, which can result in low birth weight and developmental problems. Maternal hypotension —low blood pressure in the mother— also might increase the risk of stillbirth.

Hypotension can be treated with a medication called fludrocortisone. The effects of this medicine on the fetus are not well understood. Fludrocortisone is what doctors call a mineralocorticoid, which promotes retention of fluid by the kidneys. The drug can be given along with dietary salt. Other medications, such as one called midodrine, are given specifically in cases of orthostatic hypotension; these medicines can help to keep blood pressure from dropping when you stand. However midodrine must be used with extreme caution during pregnancy. In cases of shock, various medications are available, some that increase the force of the heart contraction and others that contract blood vessels. These drugs also must be given only with extreme caution in pregnant women. This is because there is a possibility of reducing blood flow through the placenta. However, if the situation is shock (a life threatening situation in which body tissues do not receive adequate blood flow), then the goal of saving the mother’s life dominates the therapy.

As for breastfeeding,  it is unclear whether there are anti-hypotension medications used by nursing mothers can affect their nursing babies. However, there is evidence the blood pressure in mothers drops during breast feeding. Consequently, you should discuss with your doctor whether or not you should breastfeed at all, if you suffer from hypotension.

Hypotension is mild often can be managed with oral hydration (drinking adequate liquids), salt supplementation, and pressure stockings. In such mild cases, medication can be avoided. Keep in mind that hypotension is a very manageable condition. In women who become hypotensive in connection with pregnancy, often it is mild and resolves after the woman delivers her baby.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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