Antenatal anaemia: why the iron tablets are not working
Mrs Abena, 26, is 32 weeks pregnant with her second child and attends the antenatal clinic in Douala. At 28 weeks her haemoglobin was 7.8 g/dL, with small pale red cells and a low ferritin. She was given ferrous sulphate tablets, 200 mg twice a day, and told to continue them for the rest of the pregnancy. She returns four weeks later still tired and now breathless after one flight of stairs, although her haemoglobin is only 8.1 g/dL. On questioning she says she takes each tablet with a cup of strong tea because it 'settles her stomach', and she takes her calcium supplement at the same time because 'it is easier to remember'. She has also bought an expensive 'blood tonic' containing vitamin B12 and folic acid, and asks whether she should stop the iron because it makes her constipated.
1. Explain why her haemoglobin rose by only 0.3 g/dL in four weeks. Give three reasons linked to how she takes the tablets and one physiological reason. (6 marks)
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Reasons linked to absorption
- Tea. Polyphenols (tannins) in strong tea form insoluble complexes with iron in the gut, and a cup with the tablet can cut absorption by well over half. She is effectively taking a much smaller dose than prescribed.
- Calcium. Calcium is the other main inhibitor of iron absorption and competes with both haem and non-haem iron. Taken in the same swallow, it works directly against the iron.
- No vitamin C, and taken with food. Iron is absorbed best as Fe2+; ascorbic acid keeps iron reduced and soluble. Taken on an empty stomach or with a fruit juice, absorption is far greater than when taken with a meal containing phytates.
She is also probably not taking them consistently because of constipation.
The physiological reason
Even with perfect absorption, the marrow needs two to four weeks to produce new red cells, so a rise of about 1 g/dL per month is the realistic maximum. On top of that, plasma volume is expanding steeply in the third trimester, which dilutes the haemoglobin; her red cell mass may be rising while the concentration barely moves. The B12 and folate tonic does not treat iron deficiency: microcytic, hypochromic cells with a low ferritin point to iron, not to B12 or folate.
2. Explain why pregnancy increases the need for iron, and why she is breathless on the stairs even though her haemoglobin is only moderately low. (5 marks)
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Why more iron is needed
- The mother's red cell mass rises by roughly 20-30% to carry oxygen for two.
- The fetus builds its own haemoglobin and stores iron in the liver, most of it in the third trimester.
- The placenta and the blood lost at delivery (about 500 mL, more after caesarean section) must be replaced.
- Dietary intake in a typical local diet is often low in absorbable iron, and repeated pregnancies deplete stores.
Why she is breathless
Haemoglobin carries oxygen, so at 8.1 g/dL the oxygen-carrying capacity of each litre of blood is reduced by roughly a third. The body compensates by raising heart rate and cardiac output, by increasing ventilation, and by shifting the dissociation curve so that oxygen is released more readily to tissues. At rest this can be enough; on stairs the muscles' demand outruns the supply, so anaerobic respiration produces lactic acid and the drive to breathe rises sharply. The enlarging uterus also splints the diaphragm and reduces the room for the lungs to expand. Breathlessness at rest, chest pain or fainting would make this an emergency rather than a finding to manage at the next visit.
3. Advise Mrs Abena on how to take the iron and how to manage the constipation. Justify each instruction. (5 marks)
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How to take it
- Take each tablet with water or with a vitamin C-rich drink (orange, guava, baobab juice) between meals, when absorption is greatest.
- Keep tea, coffee and milk at least one to two hours away from the tablet.
- Move the calcium supplement to a different time of day, at least two hours from the iron.
- Tell her that black stools are normal and not a sign of bleeding, so she does not stop the tablets in alarm.
- Continue for at least three months after delivery to refill the stores, not just until the haemoglobin looks acceptable.
Constipation
Increase fibre (vegetables, beans, whole grains), increase water intake, and keep walking. Iron is better absorbed on an empty stomach, but if nausea is intolerable, taking it with a small amount of food is a reasonable compromise because a partly absorbed dose is better than a dose not taken at all. A stool softener can be prescribed if needed. She should not stop the iron, and should be told that the 'blood tonic' will not correct iron deficiency and may delay proper treatment. If her haemoglobin falls further or she becomes symptomatic at rest, parenteral iron or transfusion may be needed in hospital.