Maternal hypotension is defined as low blood pressure occurring during pregnancy. While the body typically adapts without issue, this temporary drop can sometimes cause noticeable symptoms. A reading below 90/60 mmHg is generally considered hypotensive. This physiological shift is a predictable part of gestation, but severe or persistent symptoms warrant attention to prevent complications for both the mother and the developing fetus.
How Pregnancy Alters Blood Pressure
The primary reason for the drop in blood pressure relates to profound adaptations of the maternal circulatory system. Early in pregnancy, hormonal changes, particularly increased progesterone, trigger widespread relaxation and widening of the blood vessels, known as peripheral vasodilation. This decrease in systemic vascular resistance means the vessels offer less resistance to blood flow, directly lowering the blood pressure.
The total volume of blood also increases significantly, sometimes by as much as 40 to 50 percent. This relative dilution of the blood, known as hemodilution, further contributes to the pressure drop. The heart must increase cardiac output to circulate this expanded volume against the lower resistance.
This decrease is typically most pronounced during the first and second trimesters, reaching its lowest point around the mid-point of the pregnancy. In the third trimester, blood pressure often begins to rise again, returning closer to pre-pregnancy levels.
Identifying the Signs
The most frequent symptoms of maternal hypotension relate directly to reduced blood flow to the brain. A pregnant individual may experience dizziness or lightheadedness, particularly when changing positions quickly. This postural, or orthostatic, hypotension occurs when the circulatory system cannot adjust fast enough after standing up.
Other indications include a general feeling of fatigue, persistent nausea, and occasional blurred vision. In more significant episodes, the individual may feel faint or experience syncope, a temporary loss of consciousness. The body’s attempt to compensate can also manifest as cold, pale, or clammy skin, along with rapid or shallow breathing.
Potential Impact on Mother and Fetus
While mild, asymptomatic low blood pressure is generally benign, severe or prolonged hypotension carries risks for both the mother and the fetus. For the mother, the primary danger comes from syncope, where fainting can lead to falls and subsequent trauma or injury. Brief episodes of severe hypotension, such as those occurring during regional anesthesia, can also cause maternal discomfort like vomiting.
The most concerning risk to the fetus is a reduction in uteroplacental perfusion, the blood flow supplying the placenta. Since blood flow to the uterus is pressure-dependent, a significant drop in maternal blood pressure decreases the delivery of oxygen and nutrients to the baby. If this low blood flow is chronic or severe, it may lead to complications like fetal distress, lower birth weight, or intrauterine growth restriction.
A specific condition known as Supine Hypotension Syndrome can occur in the second half of pregnancy. When lying flat on the back, the growing uterus compresses the inferior vena cava, a major vein returning blood to the heart. This compression significantly reduces the blood returning to the heart, causing a rapid drop in maternal cardiac output and blood pressure. The symptoms of this syndrome usually compel the individual to change position, which typically resolves the issue within minutes.
Immediate Management and Long-Term Care
The management of maternal hypotension focuses on preventative lifestyle measures and immediate symptom relief. Maintaining adequate hydration by drinking plenty of water throughout the day is an effective strategy to support blood volume. Eating small, frequent meals helps stabilize bodily functions and prevents postprandial (after-meal) drops in blood pressure.
Pregnant individuals should rise slowly and deliberately when sitting or lying down to give the circulatory system time to adjust, preventing orthostatic hypotension. Avoiding prolonged standing is also helpful, as gravity can cause blood to pool in the lower extremities. Past the halfway point of pregnancy, it is recommended to sleep or rest in a side-lying position, preferably the left side, to prevent the uterus from compressing major blood vessels.
If a hypotensive episode occurs, the immediate action is to lie down and elevate the feet above the heart to encourage blood return to the core circulation. If the blood pressure drop is severe or caused by an underlying medical condition, medical intervention may be necessary, sometimes involving intravenous fluids or specific medications. Regular prenatal monitoring allows healthcare providers to track blood pressure trends and intervene if the hypotension becomes symptomatic or concerning for fetal well-being.