Hemoglobin concentration does tend to dip during menstruation, though in most people with regular periods the drop is small enough that it stays within the normal range. The shift is driven by straightforward blood loss, but the story gets more interesting when you factor in hormonal effects on body fluid, the behavior of iron-regulating molecules across the cycle, and the surprisingly common problem of iron depletion that never gets flagged on standard lab work.
How Large Is the Drop in a Typical Cycle
Research measuring hemoglobin at different points in the menstrual cycle consistently finds a dip, but the size of that dip varies by study and is sometimes just barely detectable. One study of healthy women recorded average hemoglobin at about 13.1 g/dL in the early follicular phase (right after menstruation starts), dropping to around 12.7 g/dL in the late follicular phase and sitting at 12.8 g/dL in the late luteal phase. The difference across phases approached but did not reach statistical significance. Interestingly, total hemoglobin mass (the actual amount of hemoglobin in your body, as opposed to its concentration in a given sample of blood) did not change at all across the cycle.1The FASEB Journal. The Effect of the Menstrual Cycle on Hemoglobin Mass Another study found the difference the other way around, with hemoglobin averaging about 12.9 g/dL in the follicular phase and roughly 13.1 g/dL in the luteal phase, a statistically significant though still small variation.2Journal of Clinical and Diagnostic Research. Variation In Haemoglobin Levels During Menstrual Cycle
These numbers might look contradictory, but they actually point to the same underlying reality: the fluctuation is real but modest, generally less than half a gram per deciliter. For context, the normal hemoglobin range for adult women is roughly 12.0 to 16.0 g/dL, so a swing of 0.3 to 0.4 g/dL is a ripple, not a wave. Where these studies diverge is in timing and in whether they measured concentration or total mass, which turns out to matter quite a bit.
Why Concentration and Total Mass Tell Different Stories
You lose blood during your period, so it seems obvious that you would have less hemoglobin afterward. But your body is also shifting fluid around throughout the cycle, and that complicates the picture. Estradiol, which rises during the follicular phase, lowers the threshold at which your body triggers water-retention mechanisms. It increases plasma volume by promoting both water reabsorption and sodium retention in the kidneys.3PubMed Central. Sex hormone effects on body fluid regulation Research in postmenopausal women given estrogen replacement confirmed that estradiol ramps up the release of arginine vasopressin (the hormone that tells your kidneys to hold onto water), contributing to fluid retention when estrogen is high.4PubMed. Estrogen influences osmotic secretion of AVP and body water balance in postmenopausal women
This is why hemoglobin concentration can drop even if you haven’t lost much blood. When plasma volume expands, the same number of red blood cells is diluted in more fluid, and the concentration reading on a blood test goes down. The FASEB study’s finding that hemoglobin mass stayed constant while concentration appeared to shift is a clean illustration of this effect. In practical terms, a blood draw taken when your estrogen is peaking might return a lower hemoglobin reading than one taken right after your period ends, even though your body contains the same total amount of hemoglobin.
Iron and Hepcidin Through the Cycle
Beyond the concentration-versus-mass question, there is a real iron story playing out each month. When you menstruate, you lose iron along with the blood. A study tracking both iron and hepcidin (the liver hormone that controls how much iron your gut absorbs) found a consistent pattern: both molecules drop during menstruation, then rebound and stabilize during the second half of the cycle.5PubMed Central. Joint Model of Iron and Hepcidin During the Menstrual Cycle in Healthy Women The hepcidin drop is actually useful: lower hepcidin means the gut opens the gate wider for iron absorption, partially compensating for what was just lost.
Research in endurance-trained women added nuance, finding that while the overall effect of cycle phase on hepcidin did not reach strict statistical significance, meaningful differences in hepcidin levels appeared at specific time points. Pre-exercise hepcidin was somewhat higher in the mid-luteal phase compared to right after menstruation, and post-exercise hepcidin responses also shifted with the cycle.6PubMed Central. Menstrual cycle affects iron homeostasis and hepcidin following interval running exercise in endurance-trained women For athletes, this matters because exercise itself stimulates hepcidin, and when that spike lands at a point in the cycle where hepcidin is already elevated, iron absorption can be temporarily blunted. The practical takeaway is that timing iron intake around your period and around training sessions is more than just a wellness trend; the biology supports it, even if the effects are not dramatic in any single cycle.
How Much Blood Are You Actually Losing
A typical period involves roughly 30 to 80 mL of blood loss over several days. That is not a huge volume (a shot glass holds about 45 mL), and the iron it contains is something a well-nourished body can usually replace before the next cycle. But “typical” covers a wide range. Early radioisotope studies estimated menstrual blood losses spanning from about 33 mL in one woman with light flow up to 550 mL in women with heavier periods.7PubMed Central. THE STUDY OF MENSTRUAL AND OTHER BLOOD LOSS, AND CONSEQUENT IRON DEFICIENCY, BY FE59 WHOLE-BODY COUNTING A tenfold difference in blood loss means a tenfold difference in iron drain, and that gap explains why some people sail through their periods while others gradually slide into iron deficiency.
A large study of premenopausal blood donors quantified how much menstrual blood loss actually explains variation in hemoglobin and iron stores. Menstrual blood loss accounted for about 8% of the explained variance in hemoglobin levels, and heavy menstrual bleeding (which affected about 13% of the women studied) was associated with more than three and a half times the odds of anemia.8PubMed Central. Menstrual blood loss is an independent determinant of hemoglobin and ferritin levels in premenopausal blood donors Eight percent might not sound like much, but menstrual blood loss was the single largest independent factor in hemoglobin variation once you accounted for other variables. In the same study, women using a hormonal intrauterine device that reduces menstrual flow had dramatically lower odds of iron deficiency.
When the Drop Becomes Dangerous
For most people, the monthly hemoglobin dip is a blip that the body handles without trouble. But in some cases, heavy menstrual bleeding leads to hemoglobin levels that are outright dangerous. A study reviewing cases of severe anemia attributed to chronic menstrual bleeding found 168 episodes where hemoglobin fell below 5 g/dL, with a mean low point of 4.15 g/dL. Some women had levels as low as 1.6 g/dL, which is life-threatening.9PubMed. Severe anemia from heavy menstrual bleeding requires heightened attention These are not quick crashes; they tend to develop slowly over months or years as iron stores are depleted faster than they can be replenished.
What makes these cases insidious is that the body adapts to gradually falling hemoglobin. Someone whose hemoglobin dropped to 5 g/dL overnight would be in a medical emergency, but when it happens over many cycles, the cardiovascular system compensates (faster heart rate, increased cardiac output) and the person might feel tired or short of breath without realizing anything is seriously wrong. By the time they get a blood test, the numbers can be startlingly low.
Fibroids, Bleeding Disorders, and Other Amplifiers
Uterine fibroids are one of the most common reasons periods become heavy enough to cause meaningful hemoglobin drops. In roughly 30% of fibroid cases, the growths cause symptoms that affect quality of life, and heavy menstrual bleeding is the most frequent complaint.10PubMed. The modern management of uterine fibroids-related abnormal uterine bleeding Submucosal fibroids (those growing into the uterine cavity) are particularly linked to lower hemoglobin; one large study found they were associated with an adjusted hemoglobin level about 0.35 g/dL lower and nearly 1.5 times the odds of anemia compared to women without them.11American Journal of Obstetrics and Gynecology. Submucosal fibroids and the relation to heavy menstrual bleeding and anemia In extreme cases, fibroids have driven hemoglobin below 2 g/dL.12PubMed Central. Life-threatening anemia due to uterine fibroids: A case series
Bleeding disorders compound the problem. Von Willebrand disease, the most common inherited bleeding disorder, makes it harder for blood to clot and can turn a normal period into a heavy one. In a cohort of women with von Willebrand disease, 31% of those reporting heavy menstrual bleeding had hemoglobin below 12 g/dL, compared to just 4% of those whose periods were not heavy. Low ferritin was even more widespread: 62% of those who were still menstruating had ferritin below 30 μg/L.13PubMed Central. Prevalence of heavy menstrual bleeding, iron deficiency, iron deficiency anemia, and treatment in women with von Willebrand disease—a cohort study Many of these women had iron deficiency without formal anemia, an under-recognized state that carries its own consequences.
Iron Deficiency Without Anemia
Hemoglobin only falls below the anemia threshold once iron stores are significantly depleted. Before that happens, you can spend months or years in a state of iron deficiency that does not show up as anemia on a standard blood count. Estimates suggest that 10 to 20% of menstruating women are iron deficient, while only 3 to 5% of those are formally anemic.14PubMed Central. Iron deficiency without anemia – a clinical challenge In a study of premenopausal African-American women, 68% were iron deficient while 18% were anemic, showing how large the gap can be between depleted iron and an abnormal hemoglobin number.15PubMed Central. The association between subjective assessment of menstrual bleeding and measures of iron deficiency anemia in premenopausal African-American women: a cross-sectional study
Iron deficiency without anemia is not harmless. Subclinical depletion has been linked to impaired executive function, difficulty maintaining attention, and prolonged recovery after exercise.16Quality in Sport. Iron Deficiency and Beyond: Implications for Cognitive Function and Recovery in Female Endurance Athletes The “brain fog” and unexplained fatigue that some people chalk up to their period may be rooted in iron depletion that precedes any measurable drop in hemoglobin. If you’re only checking your hemoglobin level, you can miss this entirely.
The Ferritin Reference Range Problem
Part of the reason iron deficiency flies under the radar is that many labs use ferritin reference ranges derived from population data that include iron-deficient individuals. Ferritin is the best single blood marker for iron stores, but studies have shown that 30 to 50% of ostensibly healthy women have no iron stores in their bone marrow at all. Basing a “normal” ferritin cutoff on the lowest 2.5% of that population means setting the bar so low that genuinely depleted women are told their results are fine.17PubMed Central. Sex, lies, and iron deficiency: a call to change ferritin reference ranges
This has real consequences. A woman with a ferritin of, say, 15 μg/L might get a lab report that says “within normal range” because the reference interval bottoms out at 10 or 12 μg/L. But functionally, her iron stores are essentially empty, and her hemoglobin may be on the verge of dropping below normal. Many clinicians now consider ferritin below 30 μg/L to indicate iron deficiency regardless of what the lab printout says, and some argue the threshold should be even higher. If you are getting blood work done and your hemoglobin is technically normal but your ferritin is in the teens, that deserves attention.
Young People and the Onset of Menstruation
The iron drain starts early. A study of adolescent girls found that those who had started menstruating were 2.6 times more likely to be iron depleted than those who had not. Among the menstruating girls, about a third had ferritin levels below the depletion threshold, compared to about 16% of pre-menarchal girls. Those who were both menstruating and iron depleted had significantly lower hemoglobin, hematocrit, and serum iron.18PubMed Central. Association of Iron Depletion with Menstruation and Dietary Intake Indices in Pubertal Girls: The Healthy Growth Study This is a population that is simultaneously growing, menstruating, and often not eating iron-rich foods consistently, all of which stack up.
Adolescence also tends to be when dietary habits are most erratic, and research on this population underscores that diet alone often cannot keep pace with menstrual iron loss, particularly for those with heavier periods or low intake of heme iron from animal sources. For this age group, routine screening for iron status (not just hemoglobin) at well visits would catch depletion before it becomes anemia.
What Actually Helps
Iron supplementation is the obvious intervention, but the details of how and when to take it matter. A trial comparing iron tablets taken only during the four days of menstruation versus weekly supplementation over 16 weeks found that the weekly approach produced greater hemoglobin improvements. Both methods improved iron stores, but weekly dosing was more effective at raising hemoglobin in women who were already anemic.19PubMed. Will iron supplementation given during menstruation improve iron status better than weekly supplementation?
A Cochrane review of intermittent iron supplementation in menstruating women confirmed the broad pattern: taking iron even once or twice a week reduces the risk of anemia by roughly a quarter compared to no supplementation and raises both hemoglobin and ferritin levels. However, intermittent supplementation was associated with slightly more frequent anemia compared to daily iron, even though average hemoglobin concentrations ended up similar between the two approaches.20Cochrane Database of Systematic Reviews. Intermittent iron supplementation for menstruating women In plain terms, daily iron is the gold standard for someone who is actually anemic, but for prevention in someone with adequate hemoglobin, intermittent dosing works and tends to cause fewer side effects like nausea and constipation.
On the hormonal side, treatments that reduce menstrual bleeding can protect hemoglobin from the other direction. A systematic review found that users of levonorgestrel-releasing intrauterine devices (hormonal IUDs) saw hemoglobin increases over time, while copper IUD users, whose periods tend to become heavier, saw hemoglobin decrease.21PubMed. Hemoglobin and serum ferritin levels in women using copper-releasing or levonorgestrel-releasing intrauterine devices: a systematic review If you are choosing a contraceptive method and you already struggle with iron status, this is worth weighing. The same study from the blood-donor cohort mentioned earlier found that hormonal IUD use was associated with dramatically lower odds of iron deficiency.8PubMed Central. Menstrual blood loss is an independent determinant of hemoglobin and ferritin levels in premenopausal blood donors
An Evolutionary Angle on Menstrual Iron Loss
From a biological anthropology perspective, the framing of menstrual iron loss as inherently pathological may be overly simplistic. A review interpreting the literature on iron and women’s reproduction through an evolutionary lens pointed out that menstrual blood loss varies considerably not just between individuals but between populations, and that much of what gets labeled “iron deficiency” in modern clinical settings may partly reflect ecological variation rather than disease.22PubMed. The reproductive ecology of iron in women In ancestral environments, women spent far less of their reproductive lives menstruating. Frequent pregnancies and extended breastfeeding suppressed ovulation, meaning a woman might have had only a few dozen lifetime periods rather than the 400 or so a modern woman in a high-income country experiences. The monthly iron drain that concerns clinicians today is, in some sense, a feature of modern reproductive patterns. That does not mean iron deficiency should be ignored, but it does add context: the human body may not have been optimized by natural selection to handle 30-plus years of uninterrupted monthly blood loss without dietary or behavioral support.