Iron supplementation: lumping, splitting and choice

I’ve always kept an interested eye on the research relating to iron supplementation in pregnancy. It’s clear that some people really benefit from iron supplementation, whether pregnant or not, but it also seems to me that we have a lot of over-prescription with unclear evidence of benefit.

The authors of the 2015 Cochrane review on this topic are also aware of this. They set out, ‘To assess the effects of daily oral use of iron supplements by pregnant women, either alone or in conjunction with folic acid or with other vitamins and minerals as a public health intervention’ (Peña-Rosas et al 2015).  In contrast to older reviews (Peña-Rosas & Viteri 2009), this review specifically looks at the outcomes of daily (as opposed to intermittent) iron supplementation.

And the conclusion?

“Supplementation reduces the risk of maternal anaemia and iron deficiency in pregnancy but the positive effect on other maternal and infant outcomes is less clear. Implementation of iron supplementation recommendations may produce heterogeneous results depending on the populations’ background risk for low birthweight and anaemia, as well as the level of adherence to the intervention.” (Peña-Rosas et al 2015).

I once wrote about an older version of this review and its implications. While I have removed my articles from this site as they are out-of-date, one thing remains relevant. I included a discussion of some the complexities and uncertainties that are intrinsic to seeking evidence in this way, and within the first article I included a brief discussion of the notions of ‘lumping’ and ‘splitting’.

“If one is looking to gather as many studies – and participants – into the mix as possible, they might be said to be ‘lumping’. Their data may have more power in one sense, because they have included more women overall, but some of those women might be very, very different from each other, which means that recommendations are very broad. The review might conclude that something is either recommended or not recommended for all women, but some people may see this as not very helpful, because the results are drawn from studies carried out with women who are so different from each other that they don’t have as much meaning than if they related to populations that were a bit more homogenous.

The opposite end of this spectrum is where reviewers attempt to ‘split’ the studies and look more specifically, for instance by focusing on women having their first baby, who live in countries where malaria is endemic, who have normal Hb at the beginning of pregnancy and who are given a specific dosage of iron. It might seem that the results of such a review would be more helpful for a woman who falls into that category, than the results of a broad review, but the problems with the ‘splitting’ approach include that there might be so little data that the results are not significant, which is arguably even less helpful than a broad recommendation.” (Wickham 2013a: 29).

I also went on to write more about the philosophical issues raised by the notion of universal prophylaxis. There really are some deep and knotty issues to be debated here, and we must not forget that we do not necessarily know what is normal in this realm. I recall that the question of what constituted a normal Hb level in pregnancy was raised a number of years ago when some research showed that haemodilution meant that the cut off point was lower than we had thought, and yet this – like many other things – has not really changed anything in my area in practice. And who is to say that there is a universal ‘normal’ anyway?  Another of the issues raised by this review is the ongoing question of how we reconcile the gathering of population-level data and the making of population-level recommendations with the fact that we are caring for individual women, and this is something that we will no doubt be debating for a while…

“From whence comes the perception that evidence based practice is about seeking the one, certain, binding and universal doorway through which we will find the answer that will stand for all time?” (Wickham 2013b: 51)


I’ve also written about some of the issues that women need to consider when deciding whether and what to take in pregnancy in What’s Right For Me?, which you can read more about here.



Peña-Rosas J, De-Regil L, Garcia-Casal MN, Dowswell T. Daily oral iron supplementation during pregnancy. Cochrane Database of Systematic Reviews 2015, Issue 7. Art. No.: CD004736. DOI: 10.1002/14651858.CD004736.pub5

Wickham S (2013a). ReView: daily oral iron supplementation during pregnancy. EM 4(3):27-30.

Wickham S (2013b). Interpreting evidence. EM 4(3):50-1.

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