What Causes Low Blood Pressure in Pregnancy?

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  • Written By: Rebecca Harkin
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 08 October 2018
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The three main causes of low blood pressure in pregnancy are the increasing demand for blood during pregnancy, dehydration, and a pre-existing problem with hypotension or low blood pressure. Dizziness or fainting spells are the main impact of low blood pressure in pregnancy. These issues can be minimized by staying well hydrated and rising from a sitting or lying position slowly.

A pregnant woman experiences the greatest drop in blood pressure during the second trimester. Dependency of the growing baby on the mother for his blood supply is the cause of low blood pressure in pregnancy during this time. During the first and more so the second trimester, the mother’s blood supply must expand to accommodate this growing need, and until the body responds the shared blood supply is diminished, resulting in a lowered blood pressure. It is particularly important during the second trimester for pregnant women to take precautions to keep from becoming dizzy and falling or fainting due to the lowered blood pressure. To minimize dizziness and fainting resulting from low blood pressure in pregnancy, pregnant women should rise from a sitting or lying position slowly, preferably using a chair or wall to steady themselves, and before moving forward they should stand still for a minute to allow the body to equalize the blood pressure.


Dehydration and blood volume are directly linked. When a healthy body is adequately hydrated, the blood volume and blood pressure are normal and when the body is dehydrated, the blood volume and the blood pressure decrease. During pregnancy, the body has a greater demand for water for such needs as producing amniotic fluid and the increased blood supply. When this need is not met by drinking adequate fluids, low blood pressure in pregnancy is the result, and symptoms of dizziness and fainting will begin to appear. Many obstetricians recommend that their patients drink at least eight eight-ounce (about 236 ml) glasses of water per day.

If low blood pressure was a health problem before pregnancy, it will more than likely continue to be an issue during pregnancy, and the pregnant women may even become more hypotensive. Many of the medications used to treat low blood pressure are not recommended during pregnancy. Obstetricians will often closely monitor the low blood pressure of at-risk patients during pregnancy and try to control it by increasing water intake and moderately increasing salt intake. Increased hydration will augment the amount of water in the blood and salt will help the blood to hold onto the water, resulting in an increase in blood volume and blood pressure. If this approach fails, the benefits and risks of blood pressure medication will be examined and the best choice will be made for the pregnant woman.



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