Vitamin B12 Deficiency in Pregnancy

Vitamin B12 deficiency in pregnancy is low cobalamin, causing megaloblastic anaemia and sometimes nerve damage. Causes include vegan diet, poor diet, pernicious anaemia, gastric surgery, malabsorption, metformin, HIV and parasitic infection. It matters because it causes neural tube defects, preterm birth, recurrent miscarriage and infant developmental delay, and folic acid alone can hide the anaemia while the nerve damage worsens.

• Risk factors: vegan or vegetarian diet, previous gastric surgery, metformin, Crohn disease, autoimmune disease.
• Serum B12 falls in normal pregnancy, so use methylmalonic acid when borderline.
• Breastfed babies of deficient mothers can have severe disease.

Women have fatigue, pallor, sore tongue, tingling, numbness, poor balance and memory change, and sometimes jaundice and weight loss. Examine sensation, vibration sense, reflexes and gait.

• Peripheral neuropathy with loss of vibration sense.
• Lemon yellow skin from mild haemolysis.
• Differential diagnosis: folate deficiency, iron deficiency, thyroid disease, alcohol, HIV neuropathy, diabetes neuropathy, multiple sclerosis, drug effect.
• Complications: severe anaemia, subacute combined degeneration of the cord, neural tube defect, miscarriage, preterm birth, infant failure to thrive and developmental delay.

The approach is B12 level with methylmalonic acid and a search for cause.

• Serum vitamin B12 below 200 ng/L (below 150 pmol/L) confirms deficiency; 200 to 300 ng/L is borderline and needs serum methylmalonic acid (above 0.4 micromol/L supports deficiency) or homocysteine.
• Full blood count, MCV above 100 fL, hypersegmented neutrophils, reticulocytes, ferritin and folate.
• Intrinsic factor and parietal cell antibodies for pernicious anaemia; HIV test; stool for parasites.
• Neurological examination at each visit.

The principle is vitamin B12 replacement by injection with folic acid, and treatment of the cause.

• Supportive: diet with eggs, milk, fish and meat; fortified foods for vegans; physiotherapy and safety advice for neuropathy.
• Pharmacological: cyanocobalamin 1000 micrograms IM daily for 7 days, then weekly for 4 weeks, then monthly until delivery and lifelong for pernicious anaemia or gastric surgery; with neurological signs, give 1000 micrograms IM on alternate days until no further improvement, then every 2 months; for dietary deficiency without neurological signs use cyanocobalamin 1000 to 2000 micrograms oral daily; folic acid 5 mg oral daily after the first B12 injection; ferrous sulphate 200 mg oral 8-hourly if iron deficiency coexists; potassium chloride 600 mg oral 8-hourly if potassium is low in severe anaemia during the first week.
• Procedures: blood transfusion for haemoglobin below 5 g/dL or heart failure, using slow packed cells with furosemide 20 mg IV.
• Special groups: breastfeeding mothers continue B12; exclusively breastfed infants of deficient mothers need paediatric review and B12 treatment if signs appear; metformin users are checked yearly.
• Red flags: haemoglobin below 7 g/dL, leg weakness, loss of balance, confusion, heart failure, low potassium, fetal anomaly on scan.
• Follow-up: reticulocyte count at 1 week and haemoglobin at 2 to 4 weeks; B12 level at 3 months; neurological review each visit; lifelong B12 where indicated.