Low blood pressure, defined as a reading below 90/60 mmHg, affects women more often than men, largely because of hormonal fluctuations that directly relax blood vessels. Some causes are unique to women, like pregnancy and menstrual cycle shifts, while others, like dehydration and medication side effects, simply hit harder or more frequently in female bodies.
How Hormones Lower Blood Pressure
Progesterone, one of the two primary female sex hormones, is a natural vasodilator. It works by blocking calcium from entering the smooth muscle cells that line blood vessel walls. Since calcium is what makes those muscles contract and tighten, progesterone essentially keeps vessels relaxed and wide open. Research from the American Heart Association found that progesterone reduced calcium-driven constriction in blood vessel cells by roughly 64%. This effect happens directly at the cell membrane, meaning it kicks in quickly rather than going through the slower hormone-receptor pathway.
This is why blood pressure isn’t static across the menstrual cycle. During the menstrual and proliferative phases (roughly the first two weeks of the cycle), when progesterone is low but estrogen is rising and vessels are relatively relaxed, systolic pressure averages around 104 to 105 mmHg. In the luteal phase (the two weeks before a period), progesterone is high, but so is the body’s compensatory response, and systolic pressure climbs to about 124 mmHg. The diastolic pattern follows the same trend, from around 64 mmHg during menstruation up to 76 mmHg in the luteal phase. Women who already run on the low side may notice dizziness or lightheadedness in the first half of their cycle as a result.
Pregnancy and Blood Pressure Drops
Pregnancy is the single most dramatic cause of low blood pressure in women of reproductive age, and it starts earlier than most people expect. Blood vessels begin dilating as early as five weeks of gestation, well before the placenta is fully formed. By mid-second trimester, peripheral vascular resistance drops 35% to 40% below pre-pregnancy levels, which causes blood pressure to fall 5 to 10 mmHg below baseline. That nadir typically hits around weeks 16 to 20, though most of the initial drop happens between weeks 6 and 8.
The body tries to compensate. Cardiac output increases by up to 45% by 24 weeks. Heart rate rises 20% to 25% above normal. Plasma volume expands significantly, starting at 6 to 8 weeks and continuing to rise until around 28 to 30 weeks. But plasma volume grows faster than red blood cell production, creating a dilution effect sometimes called “physiological anemia.” This combination of wider blood vessels and thinner blood is why many pregnant women feel faint, especially when standing up quickly or lying flat on their backs.
For most women, blood pressure gradually recovers in the third trimester and returns to normal after delivery. But during those middle months, even women who have never had blood pressure issues can experience noticeable symptoms.
Dehydration and Blood Volume Loss
Your blood pressure depends on having enough fluid in your bloodstream to maintain pressure against vessel walls. When blood volume drops, pressure drops with it. Women lose blood during menstruation, which reduces volume directly. Heavy periods compound this, sometimes enough to cause symptoms like lightheadedness in the days during and after bleeding.
Beyond menstrual losses, everyday dehydration from not drinking enough water, vomiting, diarrhea, or heavy sweating during exercise all shrink blood volume. Even mild dehydration can trigger weakness, dizziness, and fatigue, particularly when standing. The body normally detects a pressure drop through sensors called baroreceptors, which signal the heart to beat faster and the blood vessels to tighten. But when fluid loss is significant, this system can’t fully compensate, and blood pressure stays low.
Orthostatic Hypotension
Orthostatic hypotension is the specific type of low blood pressure that hits when you stand up from sitting or lying down. Gravity pulls blood toward your legs, and your body is supposed to respond within seconds by tightening blood vessels and increasing heart rate. When that response is too slow or too weak, blood pressure drops and you feel dizzy, lightheaded, or like your vision is going dark.
Women are particularly susceptible during pregnancy, during the early menstrual phase, and when dehydrated. But orthostatic hypotension can also be a side effect of medications or a sign of an underlying condition affecting the nervous system. If it happens repeatedly, it’s worth tracking when it occurs and what you were doing beforehand, since the pattern often points toward the cause.
Medications That Lower Blood Pressure
Several common drug classes can cause or worsen low blood pressure. Diuretics (water pills), often prescribed for bloating or high blood pressure, work by reducing fluid volume, which directly lowers pressure. Blood pressure medications themselves can overshoot, especially if a dose was set when pressure was higher and circumstances have changed. Antidepressants, particularly older tricyclic types and some SSRIs, can interfere with the nervous system’s ability to regulate vascular tone. Beta-blockers slow heart rate, which may lower pressure too much in some women.
If you started a new medication and began feeling dizzy, fatigued, or faint within days or weeks, the timing is a strong clue. This is one of the most common and most fixable causes of low blood pressure.
Nutritional Deficiencies and Anemia
Iron, vitamin B12, and folate are all essential for producing healthy red blood cells. When any of these nutrients are insufficient, the body produces red blood cells that are either too few or too large and poorly functioning. These abnormal cells carry less oxygen, and the reduced oxygen delivery can contribute to low blood pressure along with fatigue, pale skin, and shortness of breath.
Women of reproductive age are at higher risk for iron-deficiency anemia because of monthly blood loss through menstruation. Vegetarian and vegan diets can increase the risk of B12 deficiency specifically, since B12 comes primarily from animal products. Folate deficiency is less common but can develop during pregnancy, when the body’s folate demands roughly double.
Endocrine Disorders
The adrenal glands sit on top of each kidney and produce hormones that regulate blood pressure. In adrenal insufficiency (Addison’s disease), the glands don’t make enough aldosterone or cortisol. Aldosterone controls sodium and potassium balance, and without enough of it, sodium drops too low. Since sodium helps retain water in the bloodstream, low sodium leads directly to low blood volume and low blood pressure. In severe cases, the combination of very low cortisol, low blood sugar, and low sodium can become life-threatening.
Thyroid disorders, which affect women at roughly five to eight times the rate they affect men, can also play a role. An underactive thyroid slows heart rate and weakens the heart’s pumping force. An overactive thyroid can cause blood vessels to dilate excessively. Either scenario can push blood pressure lower than normal.
Other Contributing Factors
Prolonged bed rest reduces the body’s ability to adjust to position changes, leading to drops in pressure when you finally stand. Eating large meals diverts blood to the digestive tract, which can lower pressure for one to two hours afterward, a pattern called postprandial hypotension that’s more common in older adults. Severe infections and allergic reactions cause sudden, widespread blood vessel dilation that can drop pressure dangerously fast.
Some women simply have constitutionally low blood pressure with no identifiable cause. If readings consistently run below 90/60 but you feel fine, have good energy, and aren’t experiencing dizziness or fainting, it’s generally not a problem. Low blood pressure only becomes a medical concern when it produces symptoms or drops suddenly from your personal baseline.