The most common cause of anemia in pregnancy is iron deficiency: the growing baby, placenta and expanding maternal blood volume need far more iron than many women have stored. Other causes include folate or vitamin B12 deficiency, inherited conditions such as thalassemia and sickle cell disease, blood loss, and chronic illness. A mild fall in hemoglobin is also expected because blood plasma expands faster than red cells, which is called physiological anemia of pregnancy.
In my practice, I see pregnant women referred for anemia at every stage. Most have straightforward iron deficiency that responds well to treatment, but it is always worth confirming the cause, because the fix differs.
How Pregnancy Changes the Blood
During pregnancy, the volume of blood plasma rises by roughly 40% to 50%, while the mass of red blood cells rises by a smaller proportion. The result is a dilution effect that lowers hemoglobin concentration even when iron stores are adequate. This is a normal adaptation that helps blood flow through the placenta.
Anemia is diagnosed when hemoglobin falls below the thresholds commonly used in pregnancy:
| Stage of pregnancy | Anemia if hemoglobin is below |
|---|---|
| First trimester | 11.0 g/dL |
| Second trimester | 10.5 g/dL |
| Third trimester | 11.0 g/dL |
| After delivery (postpartum) | 10.0 g/dL |
Exact cut-offs vary slightly between guidelines, so your care team will interpret your result against local standards.
The Main Causes of Anemia in Pregnancy
1. Iron Deficiency
Iron is needed to make hemoglobin, the oxygen-carrying protein in red blood cells. Pregnancy requires roughly one gram of extra iron in total, most of it in the second and third trimesters. Women who start pregnancy with low stores, often because of heavy periods, a diet low in iron, or a recent previous pregnancy, easily run short.
2. Folate Deficiency
Folate is needed for cell division, and demand rises sharply in pregnancy. Deficiency causes large, immature red cells (megaloblastic anemia). Folic acid supplements also reduce the risk of neural tube defects in the baby, which is why they are recommended before conception and in early pregnancy.
3. Vitamin B12 Deficiency
B12 deficiency is less common but important, particularly in women following a vegan diet without fortified foods or supplements, and those with pernicious anemia or previous stomach or bowel surgery.
4. Inherited Hemoglobin Disorders
Thalassemia and sickle cell disease, and even carrier states, can cause or worsen anemia. They are more common in people with Mediterranean, Middle Eastern, South Asian, Southeast Asian, African and Caribbean ancestry. Thalassemia trait can mimic iron deficiency, so testing matters before giving large doses of iron.
5. Blood Loss
Bleeding in early pregnancy, placental problems later on, and bleeding at delivery can all cause anemia. Hidden losses from the gut, such as from hemorrhoids, are also possible.
6. Chronic Illness and Infection
Kidney disease, inflammatory bowel disease and other long-term conditions can reduce red cell production. In some parts of the world, malaria and hookworm infection are major causes of anemia in pregnancy.
7. Other Contributing Factors
- Twin or multiple pregnancy
- Pregnancies spaced closely together
- Severe morning sickness (hyperemesis gravidarum), limiting food intake
- Adolescent pregnancy, when the mother’s own needs are still high
- Previous weight-loss surgery affecting absorption
Symptoms and Why They Are Easy to Miss
Many symptoms of anemia overlap with normal pregnancy, which is why routine blood tests are so valuable. Look out for:
- Tiredness beyond what you would expect
- Breathlessness, palpitations or a racing heart
- Pale skin, lips or nail beds
- Dizziness or feeling faint
- Headaches and difficulty concentrating
- Cravings for ice or non-food items (pica), a classic clue for iron deficiency
- Restless legs
Diagnosis, Risks and Treatment
A full blood count is checked at the first antenatal visit and again later in pregnancy, typically around 28 weeks. If hemoglobin is low, doctors check ferritin (iron stores), and sometimes folate, B12 and hemoglobin studies for thalassemia or sickle cell disease.
Why Treatment Matters
Untreated anemia, especially iron deficiency, is associated with preterm birth, low birth weight, increased tiredness, reduced tolerance of blood loss at delivery, and a higher chance of needing a blood transfusion. It can also contribute to postnatal fatigue and low mood.
How It Is Treated
- Oral iron is first-line for iron deficiency. Taking it on an empty stomach or with vitamin C helps absorption, and doses on alternate days may be better tolerated.
- Intravenous iron is used when tablets are not tolerated, not effective, or anemia is found late in pregnancy, typically after the first trimester.
- Folic acid or vitamin B12 for confirmed deficiencies.
- Specialist care for women with thalassemia or sickle cell disease.
- Blood transfusion is reserved for severe anemia or major bleeding.
Your doctor will usually recheck hemoglobin a few weeks after starting iron to confirm it is rising.
Prevention
- Take folic acid before conception and in early pregnancy.
- Eat iron-rich foods such as lean red meat, poultry, fish, beans, lentils, tofu, fortified cereals and dark leafy greens.
- Pair plant-based iron with vitamin C and avoid tea or coffee with meals.
- If you are vegan or vegetarian, ensure a reliable B12 source.
- Consider carrier testing if hemoglobin disorders run in your family.
For a broader overview of anemia types and treatments, see our complete anemia guide.
When to See a Doctor
Tell your midwife or doctor if you have marked tiredness, breathlessness, palpitations or dizziness. Seek urgent care for chest pain, fainting, a very fast heartbeat at rest, heavy vaginal bleeding, or severe shortness of breath.
Frequently Asked Questions
Is some drop in hemoglobin normal during pregnancy?
Yes. Plasma volume rises faster than red cell mass, so hemoglobin concentration falls slightly. Anemia is diagnosed only when it drops below the pregnancy-specific thresholds.
Which trimester is anemia most common in?
It is most common in the second and third trimesters, when the baby’s iron needs and the expansion of maternal blood volume are greatest.
Can anemia in pregnancy harm the baby?
Significant iron deficiency anemia is linked with preterm birth and low birth weight. Treating it promptly lowers these risks and helps the mother cope with blood loss at delivery.
Should all pregnant women take iron supplements?
Recommendations vary by country. Many advise testing and treating women found to be anemic or iron deficient, while some recommend routine low-dose iron. Follow the advice of your care team.
