Pregnancy demands roughly 1,000 milligrams of iron over 40 weeks, nearly triple what your body normally needs. Most women don’t enter pregnancy with enough stored iron to meet that demand, which is why iron deficiency affects an estimated 37% of pregnant women worldwide. Understanding why iron drops so sharply during pregnancy helps explain why this is the single most common nutritional deficiency in expectant mothers.
Your Blood Volume Nearly Doubles
The biggest driver of low iron in pregnancy is something your body does on purpose: it floods your bloodstream with extra plasma. Maternal plasma volume expands by about 45% on average to supply oxygen and nutrients to the uterus, placenta, and growing baby. Red blood cell production ramps up too, but not nearly as fast. The result is that your blood becomes more diluted, and hemoglobin concentration naturally drops. This is sometimes called “physiological anemia of pregnancy” because it happens even in well-nourished women.
Building all those extra red blood cells requires iron. Your bone marrow pulls from your iron stores (measured by a protein called ferritin) to manufacture hemoglobin, the oxygen-carrying molecule inside each red cell. If your stores were modest before conception, this surge in demand can drain them quickly. Ferritin levels below 30 are generally enough to confirm iron deficiency, yet the lower end of pregnancy reference ranges dips as low as 5, which shows just how depleted stores commonly become.
The Fetus Takes What It Needs
Your baby is not a passive recipient. The placenta actively transports iron from your bloodstream to the fetal circulation, and the fetus requires approximately 270 milligrams of iron over the course of pregnancy to build its own blood supply, muscles, and developing organs. The placenta itself also uses iron for its growth and function. This transfer is largely one-directional: iron crosses to the baby, but it doesn’t come back. Even when your own stores are running low, the placenta continues pulling iron to protect fetal development, which means your levels drop further before your baby’s do.
Daily Requirements Jump Sharply
Outside of pregnancy, most adult women need about 18 milligrams of iron per day. During pregnancy, that number rises to 27 milligrams per day, according to recommendations from the American College of Obstetricians and Gynecologists. That 50% increase is difficult to meet through diet alone, especially if you rely primarily on plant-based iron sources, which your body absorbs less efficiently than the iron found in meat, poultry, and fish.
Many women don’t realize they were already falling short before becoming pregnant. Years of menstrual blood loss, combined with a diet that doesn’t fully replace those losses, means a significant number of women conceive with low iron stores. Pregnancy then accelerates the deficit rather than creating it from scratch.
Your Body Adjusts Absorption, but It Has Limits
Pregnancy triggers a clever hormonal shift to help compensate. A liver hormone called hepcidin, which normally acts as a gatekeeper controlling how much iron enters your bloodstream from food, drops significantly during the second and third trimesters. Lower hepcidin means your gut absorbs more iron from each meal. This is your body’s attempt to keep up with rising demand.
But this adaptation has limits. If your diet is low in iron-rich foods, there’s simply not enough iron in the gut for your body to absorb, no matter how wide it opens the gates. Nausea and vomiting compound the problem. If you can’t keep food down during the first trimester, you miss a critical window for building iron stores before the heaviest demands of later pregnancy kick in.
Iron supplements can help, but timing matters. Research suggests that taking iron supplements rapidly raises hepcidin levels, which temporarily reduces absorption efficiency. This means a large daily dose may not be absorbed as well as smaller or less frequent doses, though your provider can help you find the right approach.
Risk Factors That Stack the Odds
Some women are more vulnerable to low iron during pregnancy than others. The most common risk factors include:
- Closely spaced pregnancies. Your body needs time to rebuild iron stores after delivery and breastfeeding. Getting pregnant again within a short window means you start the next pregnancy already depleted.
- Carrying multiples. Twins or triplets mean a larger placenta, more blood volume expansion, and more iron transferred to multiple babies.
- Severe morning sickness. Frequent vomiting limits how much iron you absorb from food and supplements.
- Heavy periods before pregnancy. Chronic menstrual blood loss is one of the most common reasons women enter pregnancy with low ferritin.
- A diet low in iron-rich foods. Vegetarian and vegan diets, or diets that rely heavily on processed foods, often fall short of the 27-milligram daily target.
- Pre-existing anemia. If you were already iron-deficient before conceiving, pregnancy will deepen the deficit rapidly.
Underlying Conditions That Block Absorption
Sometimes iron intake is adequate, but the body can’t absorb it properly. Celiac disease is one of the most common culprits. When someone with undiagnosed celiac disease eats gluten, the lining of the small intestine becomes damaged, reducing its ability to absorb nutrients including iron. Poorly controlled celiac disease during pregnancy also raises the risk of delivering a baby with low birth weight.
Gastric bypass and other bariatric surgeries physically alter the digestive tract, bypassing the section of the small intestine where most iron absorption occurs. Women who have had these procedures often need higher-dose or alternative forms of iron supplementation throughout pregnancy. Inflammatory bowel conditions like Crohn’s disease can cause similar absorption problems, particularly when the disease is active in the upper small intestine.
Why Low Iron Matters for Pregnancy Outcomes
Iron deficiency isn’t just an inconvenience. It progresses to iron-deficiency anemia when stores drop low enough that your body can no longer produce adequate hemoglobin. At that point, less oxygen reaches your tissues and your baby’s. Research published in The American Journal of Clinical Nutrition found that anemia during pregnancy was associated with roughly double the risk of preterm birth. Babies born early face a cascade of health challenges, and preventing that outcome is one of the key reasons iron status is monitored throughout pregnancy.
Low maternal iron also affects how much iron your baby is born with. Newborns rely on the iron stores they built in utero to sustain them through the first several months of life, since breast milk contains relatively little iron. A mother who is significantly iron-depleted may not be able to provide those reserves fully, which can affect the baby’s early development.
For you, the symptoms of low iron tend to creep in gradually: fatigue that feels heavier than normal pregnancy tiredness, shortness of breath with mild activity, dizziness, pale skin, and difficulty concentrating. These overlap with common pregnancy complaints, which is part of why iron deficiency often goes unrecognized until bloodwork reveals it.