Low blood pressure during pregnancy is primarily caused by the dramatic expansion of your circulatory system and the hormonal shifts that relax your blood vessel walls. Most pregnant women experience some degree of blood pressure drop, with readings typically reaching their lowest point around 22 to 24 weeks of gestation. While this is a normal part of pregnancy, certain factors can push blood pressure low enough to cause dizziness, fainting, or other symptoms worth understanding.
How Pregnancy Lowers Blood Pressure
The single biggest driver is a steep drop in what doctors call vascular resistance, which is essentially how tightly your blood vessels are squeezed. Starting as early as six weeks into pregnancy, your blood vessels begin to relax and widen. Estrogen and progesterone trigger the release of several natural compounds that dilate blood vessel walls, including nitric oxide and prostacyclin. The result is a 25 to 30 percent fall in the overall resistance your heart pumps against.
Your body compensates by increasing cardiac output by about 40 percent and expanding blood volume by roughly 45 percent, adding 1,200 to 1,600 milliliters of extra blood by late pregnancy. But the expansion of vessel capacity outpaces the increase in blood volume. About 85 percent of that extra volume pools in the venous side of your circulation, creating what researchers describe as a state of “arterial under-filling.” Your arteries essentially have more space than blood to fill them, and blood pressure drops as a consequence.
When Blood Pressure Is Lowest
Blood pressure begins falling in the first trimester and generally hits its lowest point around 22 to 24 weeks, which is late in the second trimester. From about 28 weeks onward, it gradually climbs back up and typically returns to your pre-pregnancy level by around 36 weeks. This pattern holds for most clinically healthy pregnancies, though women who started with lower baseline blood pressure may notice symptoms earlier or more intensely.
In research settings, persistent hypotension in pregnancy is often defined as a systolic reading below 100 mmHg or a diastolic reading below 60 mmHg across multiple prenatal visits. Occasional dips below those numbers are common and usually harmless. Persistent readings in that range, particularly in the third trimester, deserve closer monitoring.
Positional Blood Pressure Drops
One of the most recognizable causes of sudden low blood pressure in pregnancy is lying flat on your back. As the uterus grows, it sits at the same level as the large vein (the inferior vena cava) that returns blood from your lower body to your heart. When you lie on your back, the weight of the uterus compresses this vein against the spine, drastically reducing blood flow back to the heart. Blood pressure can plummet within minutes, causing lightheadedness, nausea, or even fainting.
This is sometimes called supine hypotensive syndrome, and it resolves quickly when you roll onto your side, which lifts the uterus off the vein. It becomes more common as pregnancy progresses and the uterus gets heavier. Sleeping on your left side is the standard recommendation to keep this from happening overnight.
Dehydration and Anemia
Your blood volume needs to expand dramatically during pregnancy, so anything that limits that expansion can worsen low blood pressure. Dehydration is one of the most common and fixable culprits. When fluid intake doesn’t keep pace with the demands of an expanding circulatory system, blood volume falls short and pressure drops.
Iron deficiency anemia, which is extremely common in pregnancy, also plays a role. When red blood cell production can’t keep up with the rapid increase in plasma volume, hemoglobin concentration falls. Severe anemia is associated with a fast heartbeat, low blood pressure, and difficulty concentrating. Because plasma naturally expands faster than red blood cell mass during pregnancy, some degree of dilutional anemia is expected, but true iron deficiency makes it significantly worse.
Less Common Medical Causes
Most pregnancy-related low blood pressure is a normal physiological response, but a few medical conditions can cause or worsen it. Adrenal insufficiency (Addison’s disease) causes severe weakness, very low blood pressure, and low blood sugar. Diagnosing it during pregnancy is tricky because its hallmark symptoms, including nausea, vomiting, fatigue, and poor appetite, overlap heavily with normal pregnancy symptoms. Standard lab tests may not clearly distinguish mild adrenal insufficiency from typical hormonal changes.
Thyroid disorders and certain heart conditions can also contribute to abnormally low blood pressure. These are uncommon but worth considering if your symptoms are severe or don’t follow the typical pattern of improving in the third trimester.
Sudden Blood Pressure Drops as a Warning Sign
A sudden, sharp drop in blood pressure during early pregnancy can signal an ectopic pregnancy that has ruptured. When a pregnancy implants in the fallopian tube and the tube bursts, it causes internal bleeding that can be life-threatening. Symptoms include severe abdominal pain, fainting, low blood pressure, and shoulder pain. This is a surgical emergency.
How Low Blood Pressure Affects the Baby
Occasional dizziness from low blood pressure is unlikely to harm your baby. Persistent hypotension, however, may reduce blood flow to the placenta over time. A large study of first-time mothers found that women with persistent hypotension (low readings across multiple prenatal visits in the first two trimesters) were significantly more likely to have smaller-than-expected babies. About 21 percent of women with persistent hypotension had small-for-gestational-age newborns, compared to about 12 percent of women without it. The risk was 65 percent higher even after accounting for other factors.
Importantly, transient low blood pressure, meaning it showed up at only one or two visits, was not linked to smaller babies. The concern is specifically about chronically low readings that suggest ongoing reduced blood flow to the placenta.
Managing Symptoms Day to Day
Since most pregnancy-related low blood pressure stems from normal cardiovascular changes, management focuses on reducing symptoms rather than treating a disease. Staying well hydrated is the most effective first step, as your body needs substantially more fluid than usual to fill an expanded circulatory system. Small, frequent meals help prevent the blood pressure dips that can follow large meals, when blood flow is diverted to digestion.
Standing up slowly gives your body time to adjust to the shift in blood distribution. Compression stockings can help prevent blood from pooling in your legs, which is already exaggerated during pregnancy. Avoiding long periods of standing and lying on your left side rather than your back address two of the most common positional triggers. If you feel dizzy, sitting or lying on your side usually resolves it within a few minutes.
Moderate salt intake supports blood volume during pregnancy, though this is a balancing act. Excessive sodium can contribute to fluid retention and, in some women, raise the risk of high blood pressure complications later in pregnancy. Most women can follow their natural salt cravings without overthinking it.