Iron deficiency is the commonest nutritional problem in pregnancy, and haemoglobin is a late marker of it. By the time the haemoglobin drops, stores have been empty for some time, and the woman has already spent weeks with the fatigue, breathlessness and reduced exercise tolerance that get attributed to being pregnant.
So check ferritin at booking alongside the full blood count, not only when the haemoglobin is low. A ferritin below 30 micrograms per litre indicates deficiency in pregnancy even with a normal haemoglobin, and it is the number that tells you whether a low-ish haemoglobin is iron deficiency or dilution. Remember that ferritin is an acute phase reactant, so a normal value with raised inflammatory markers does not exclude deficiency.
Then treat properly rather than symbolically. Oral iron on alternate days is absorbed better than daily dosing and is tolerated better, which matters because the commonest reason oral iron fails is that it was never taken. Recheck at four weeks; a rise of less than 10 g/L in haemoglobin means either non-adherence, malabsorption, or another cause, and it is the point at which intravenous iron should be considered rather than a third oral prescription.
- Check ferritin at booking with the full blood count, not only when haemoglobin is low.
- Treat a ferritin below 30 micrograms per litre as deficiency in pregnancy, whatever the haemoglobin.
- Interpret ferritin against inflammatory markers - it rises with inflammation and can mask deficiency.
- Prescribe oral iron on alternate days rather than daily; absorption and tolerance are both better.
- Recheck at four weeks and escalate to intravenous iron if haemoglobin has risen by less than 10 g/L.
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