Megaloblastic Anaemia in Pregnancy
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Direct answer
Megaloblastic anaemia in pregnancy is most often caused by folate deficiency, with vitamin B12 deficiency less common but important, and both produce macrocytosis with hypersegmented neutrophils on the peripheral smear. Folate deficiency raises the risk of neural tube defects, so supplementation begins before conception. Oral folic acid treats folate deficiency, while intramuscular vitamin B12 is needed when B12 is low, and treatment is guided by serum levels.
What you must remember
- Folate deficiency: the commonest cause in pregnancy, with serum folate below about 3 ng/mL in many laboratories.
- B12 deficiency: raises methylmalonic acid and homocysteine, and is linked to vegetarian diets, malabsorption and pernicious anaemia.
- Smear: macrocytes and hypersegmented neutrophils with five or more lobes, though MCV can stay normal in mixed iron and folate deficiency.
- Neural tube defects: periconceptional folic acid lowers their risk, so preconception intake matters.
- Folate treatment: oral folic acid, commonly 5 mg daily, until the haematological response is established.
- B12 treatment: intramuscular hydroxocobalamin or cyanocobalamin in standard regimens, started when B12 deficiency is suspected because folate alone can mask it.
- Drug causes: methotrexate, trimethoprim and phenytoin impair folate metabolism.
A diagnostic and treatment pathway
A 24-year-old vegetarian at 18 weeks has haemoglobin of 8.9 g/dL and an MCV of 112 fL. The pathway separates folate from B12 deficiency, because treating the wrong one can mask a neurological problem while the blood count recovers.
- Confirm the anaemia: full blood count with MCV and reticulocyte count, noting that MCV above about 100 fL is a clue.
- Examine the smear: look for macrocytes, hypersegmented neutrophils and any coexisting iron-deficient red cells.
- Measure the levels: serum folate and B12, with methylmalonic acid and homocysteine when B12 is borderline.
- Treat by cause: folic acid for folate deficiency, intramuscular B12 when low, and iron if iron deficiency coexists.
- Monitor the response: reticulocytes rise within several days and haemoglobin climbs over weeks, so a poor response prompts review.
- Prevent recurrence: continue folate supplementation through pregnancy and advise folic acid before any future conception.
A reticulocyte response within about a week is the clearest early sign that treatment is working.
Where the exam tries to catch you
Megaloblastic anaemia questions often hinge on checking B12 before treating with folate, and on the limits of the MCV.
- Trap: "Give folic acid alone for macrocytic anaemia without checking B12." Correct: folate can correct the blood count while B12 neuropathy progresses, so both must be checked.
- Trap: "A normal MCV excludes megaloblastic anaemia." Correct: mixed iron and folate deficiency can normalise the MCV, so the smear and serum levels matter.
- Trap: "Folate reduces neural tube defects only after conception." Correct: neural tube closure occurs very early, so folic acid is started before conception.
Frequently asked questions
What is the commonest cause of megaloblastic anaemia in pregnancy?
Folate deficiency is the commonest cause of megaloblastic anaemia in pregnancy, driven by increased demand and poor dietary intake. Vitamin B12 deficiency is also important, particularly in vegetarians, so both should be checked.
Why must vitamin B12 be checked before treating megaloblastic anaemia with folic acid?
Vitamin B12 must be checked because folic acid can correct the blood count while B12 deficiency progresses silently to neurological damage. Serum B12, methylmalonic acid and homocysteine help separate the two deficiencies before treatment.
What folic acid dose is used to treat megaloblastic anaemia in pregnancy?
Treatment of folate-deficient megaloblastic anaemia commonly uses oral folic acid at 5 mg daily until the haematological response is established. Preventive doses for neural tube defects are much lower, and the local protocol should be followed.
How does megaloblastic anaemia appear on a peripheral smear?
A peripheral smear in megaloblastic anaemia shows macrocytes and hypersegmented neutrophils, with five or more nuclear lobes. These findings, with macrocytosis above about 100 fL, point towards folate or B12 deficiency rather than iron deficiency.