Anemia during the third trimester of pregnancy is diagnosed when hemoglobin falls below 11 g/dL, the cut-off widely used by the World Health Organization and U.S. obstetric guidance. A mild drop is expected because blood plasma expands faster than red cells, but a level under 11 g/dL usually reflects a real shortage, most often of iron. Because delivery and its blood loss are only weeks away, late-pregnancy anemia is worth finding and treating promptly.
As a hematologist, I see many referrals in the last trimester. The good news is that most cases respond well to treatment when it starts early enough.
Why Hemoglobin Falls in Late Pregnancy
During pregnancy, the volume of blood plasma rises substantially, while the mass of red blood cells rises by a smaller proportion. The blood becomes more dilute, a normal change called physiological anemia or hemodilution. It helps the placenta receive good blood flow and protects against blood loss at birth.
The dilution peaks in the second trimester, which is why cut-offs differ by stage. At the same time, demand for iron climbs steeply in the third trimester as the baby builds its own iron stores and the mother’s red cell mass grows.
| Stage of pregnancy | Hemoglobin defining anemia |
|---|---|
| First trimester | Below 11.0 g/dL |
| Second trimester | Below 10.5 g/dL (U.S. guidance) |
| Third trimester | Below 11.0 g/dL |
| Severe anemia in pregnancy | Below 7.0 g/dL (WHO) |
Causes and Risk Factors
Iron-deficiency anemia is by far the most common cause. Many women start pregnancy with low iron stores after years of menstruation, and diet alone often cannot meet the extra requirement of late pregnancy. Folate deficiency is the next most common nutritional cause, and vitamin B12 deficiency occurs occasionally, particularly with vegan diets or previous weight-loss surgery.
Risk factors include:
- Pregnancies spaced closely together
- Twins or other multiple pregnancies
- Heavy periods before pregnancy, or anemia already present early on
- Severe morning sickness limiting food intake
- Diets low in meat, fish or iron-fortified foods
- Inherited conditions such as thalassemia trait or sickle cell disease
- Infections such as malaria or hookworm in regions where they are common
Symptoms in the Third Trimester
Mild anemia often causes no symptoms at all and is found on routine blood tests. As it worsens, it can cause fatigue, weakness, dizziness, headaches, shortness of breath, pallor, palpitations and restless legs. Some women crave ice or other non-food items, a symptom called pica.
The difficulty is that normal late pregnancy also causes tiredness and breathlessness. That overlap is why routine blood testing matters more than symptoms at this stage.
Why It Matters Before Delivery
Anemia in late pregnancy reduces the reserve a mother has for the blood loss of birth. Even an average delivery involves some bleeding, and a woman who starts with low hemoglobin is more likely to need a transfusion if bleeding is heavier than expected.
Moderate to severe maternal iron-deficiency anemia has been associated with preterm birth and low birth weight. After delivery, it contributes to exhaustion, slower recovery, difficulty with breastfeeding and low mood. Babies born to iron-deficient mothers may also have smaller iron stores in early life.
Diagnosis and Testing
Most units check a complete blood count (CBC) at booking and again around 28 weeks, with further tests if the result is low. The CBC shows hemoglobin, hematocrit and red cell indices such as MCV.
- Serum ferritin is the most useful test for iron stores; a low value confirms iron deficiency.
- Vitamin B12 and folate are checked when red cells are large or diet suggests risk.
- Hemoglobin electrophoresis is used if thalassemia or sickle cell trait is possible, especially when cells are small but ferritin is normal.
- Blood smear helps when hemolysis or another blood disorder is suspected.
In practice, many clinicians start a trial of oral iron when a pregnant woman has anemia with small cells, then recheck hemoglobin within a few weeks. A good rise confirms iron deficiency; no rise prompts further investigation.
Treatment and Management
Oral iron is first-line. It works best taken on an empty stomach or with vitamin C, and away from tea, coffee, calcium and antacids. Alternate-day dosing can improve absorption and reduce side effects such as nausea and constipation for some women. Dark stools are expected.
In the third trimester, time is short. Intravenous iron is considered when hemoglobin is low late in pregnancy, when tablets are not tolerated, or when there is no response. It is generally avoided in the first trimester but is used in the second and third.
Other steps depend on the cause: folic acid for folate deficiency, B12 replacement, and treatment of infections. Iron-rich foods such as red meat, poultry, fish, legumes and fortified cereals support treatment but rarely correct established anemia alone. Transfusion is reserved for severe anemia or active bleeding. For a broader overview of approaches, see our article on anemia management.
Your birth team should know your latest hemoglobin before labor. If anemia persists, delivery is often planned with blood available, and the third stage of labor is actively managed to limit blood loss.
When to See a Doctor
Contact your midwife or doctor if you notice worsening breathlessness at rest, chest pain, a racing heart, fainting, or extreme fatigue that stops normal activity. Report any vaginal bleeding immediately. If you are on iron and still feel unwell, ask for a repeat blood count rather than waiting for the next scheduled test.
Frequently Asked Questions
Is some drop in hemoglobin normal in the third trimester?
Yes, a mild fall is expected because plasma volume expands. The concern starts when hemoglobin drops below 11 g/dL, which usually signals true iron deficiency rather than simple dilution.
Can third-trimester anemia harm my baby?
Mild anemia rarely causes problems. More significant anemia is linked to preterm birth and low birth weight, and can leave the baby with lower iron stores, so it is worth treating.
How quickly will iron raise my hemoglobin?
With oral iron, hemoglobin usually starts rising within two to three weeks. Intravenous iron can work faster, which is why it is often chosen close to delivery.
Can diet alone fix anemia in the third trimester?
Usually not once anemia is established, because the iron needed in late pregnancy is more than food can supply in a few weeks. Iron-rich foods and prenatal vitamins help prevent anemia and support treatment, but supplements are normally required.
Should I keep taking iron after the baby is born?
Often yes. Most women are advised to continue for several weeks to three months after delivery to rebuild iron stores; your team will guide this with a follow-up blood test.
