Blood flow goes through considerable changes throughout pregnancy.
Between weeks 6 and 8 of pregnancy, the body starts producing more blood. It continues to do this right through the third trimester, with overall blood volume increasing by roughly 50%. This large increase in blood allows for uteroplacental circulation, where oxygen and nutrients travel between the uterus and the placenta, helping nutrients reach the fetus and then filtering waste from the placenta. More blood volume also helps the body compensate for blood loss during delivery.
Heart rate and blood pressure
In order to compensate for the increases in blood flow, the heart rate increases as well, in order to raise cardiac output (the volume of blood that the heart pumps per minute). In the first trimester, cardiac output rises until it is 30 to 40% higher than it is in a non-pregnant state. You can expect to notice heart rate changes of around 10-15 beats per minute.
The body then releases progesterone to relax the veins so they can support more blood flow. The increased blood pressure puts stress on the veins which can cause varicose veins.
It’s not abnormal for blood pressure to drop in the first half of pregnancy, as the body releases hormones that dilate the blood vessels, which causes a decrease in blood pressure. However, blood pressure may increase again in the second half of pregnancy due to increased blood volume. Your healthcare provider will probably take your blood pressure at every prenatal appointment to check that it’s within a healthy range.
- Edema, or swelling due to increased water retention, is a normal pregnancy side effect and most commonly affects the face, hands, feet, legs, and ankles.
- Dizziness or lightheadedness, because it takes more time for blood to return to the brain and other parts of the body since the body is now also making its way through the placenta.
Blood pressure fluctuations in pregnancy are often safe, but occasionally people can experience certain complications that result from the changes in their body’s blood flow during gestation. The most common complications are as follows:
- Preeclampsia and eclampsia, which are serious pregnancy conditions diagnosed after 20 weeks gestation, and affect other organs like the kidney and liver.
- Gestational hypertension or high blood pressure. Those with gestational hypertension will not have protein in the urine or issues with the kidneys or heart. It usually resolves itself after birth, but can develop into chronic hypertension.
- Blood clots are more likely to arise during pregnancy, in part because of decreased circulation of blood flow, especially in the lower extremities, as well as clotting factors that make blood clots more likely.
Blood flow changes during pregnancy are a necessary component of healthy fetal development and maternal functioning. Despite their normalcy, it’s important for healthcare providers to closely monitor blood pressure and watch for any symptoms that could indicate potential complications for the parent-to-be and their growing fetus.
- “Aspirin Use to Prevent Preeclampsia and Related Morbidity and Mortality: Preventive Medication.” USPSTF. USPSTF. September 28, 2021. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/low-dose-aspirin-use-for-the-prevention-of-morbidity-and-mortality-from-preeclampsia-preventive-medication#tab1.
- Hermida RC, Ayala DE, Fernández JR, Mojón A, Alonso I, Silva I, et al. Administration time-dependent effects of aspirin in women at differing risk for preeclampsia. Hypertension. (1999) 34:1016–23. doi: 10.1161/01.hyp.34.4.1016.
- F Hytten. “Blood volume changes in normal pregnancy.” Clin Haematol. 14(3):601-12. Web. Oct 1985.
- Mayo Clinic. “Ch. 6. Part 3. Pregnancy, Childbirth and the Newborn: The Complete Guide.” Mayo Clinic Guide To A Healthy Pregnancy. Web. Oct 2011.
- Ginny Butler. “Circulation during pregnancy.” PNMag. Pregnancy and Newborn Magazine, n.d. Web.
- Monika Sanghavi, John D. Rutherford. “Cardiovascular Physiology of Pregnancy.” Circulation. 130:1003-1008. Web. Sep 2014.