Anaemia and Iron Infusions in Pregnancy

The point at which tablets stop being the answer

Most iron deficiency in pregnancy is diagnosed on a routine blood test and treated with oral iron. This page is about what happens next when that does not work — when the haemoglobin has not moved after weeks of tablets, when the side effects make them impossible to take, or when there is not enough time left before birth for oral iron to do anything.

The thresholds that define anaemia in pregnancy are set out by NICE Clinical Knowledge Summaries: "In pregnant women — Hb below 110 g/L throughout pregnancy. An Hb level of 110 g/L or more appears adequate in the first trimester, and a level of 105 g/L appears adequate in the second and third trimesters. Postpartum — below 100 g/L."

Ferritin is the test that confirms the store is empty rather than the count being diluted. CKS states that "in all people, a serum ferritin level of less than 30 micrograms/L confirms the diagnosis of iron deficiency", while adding two caveats that matter in pregnancy: levels are difficult to interpret if infection or inflammation is present, and "ferritin levels may be less reliable in pregnancy".

Why oral iron fails often enough to matter

Oral iron is poorly absorbed at the best of times, and pregnancy adds problems. Nausea, constipation, metallic taste, black stools and abdominal pain lead a large number of women to stop taking it. Absorption is also blunted by tea, coffee, calcium and some medicines taken at the same time, and hepcidin — the hormone that regulates iron absorption — rises after each dose, so more frequent dosing does not straightforwardly mean more iron absorbed.

None of that is a failure of effort. If your haemoglobin has not risen after a reasonable trial of tablets taken as prescribed, that is a clinical finding, and it is the finding that leads to the infusion conversation.

What the MHRA restricts, and why the first trimester is excluded

Intravenous iron in pregnancy is governed by a specific MHRA Drug Safety Update, and its content explains several things patients are told without explanation.

On the first trimester: "Iron-deficiency anaemia in the first trimester of pregnancy can usually be treated with oral iron (ie, IV iron should not be used). Later in pregnancy, any benefits of using IV iron should be carefully weighed against the risks: anaphylactic or anaphylactoid reactions could have serious consequences for both mother and foetus."

On when it may be used: "IV iron should not be used during pregnancy unless clearly necessary. Treatment should be confined to the 2nd or 3rd trimesters, if the benefit is clearly judged to outweigh the potential risks for both mother and foetus."

On the setting: "IV iron should only be given in an environment where the patient can be adequately monitored, and where resuscitation facilities are available." Elsewhere the same update requires that products "only be administered when staff trained to evaluate and manage anaphylactic or anaphylactoid reactions — as well as resuscitation facilities — are immediately available".

On test doses: the MHRA withdrew the old practice. "An initial test dose on first use of an IV iron product for a patient is no longer recommended," because there are no clear data that it minimises risk and "it may give false reassurance because hypersensitivity reactions have been reported in patients that had a negative initial test dose".

The risk, stated plainly

The MHRA describes "serious hypersensitivity reactions, including life-threatening and fatal anaphylactic and anaphylactoid reactions" as reported with intravenous iron. It stresses that these "can occur even when a previous administration has been tolerated (including a negative test dose)" and that "caution is therefore needed with every dose of IV iron, even if previous administrations have been well tolerated".

Risk is higher in people with known allergies including drug allergies, with immune or inflammatory conditions such as systemic lupus erythematosus or rheumatoid arthritis, or with a history of severe asthma, eczema or other atopic allergy. Tell the team about all of these before the cannula goes in.

These reactions are rare. The reason the whole infusion happens in a monitored setting with a nurse nearby is that when they do occur they need treating within seconds.

What the day itself involves

You will usually be in a day unit or an antenatal ward bay rather than admitted. A cannula goes into a vein, the iron is given as an infusion, and you stay afterwards for observation — the MHRA asks that patients "be closely monitored for signs of hypersensitivity during and for at least 30 minutes after every administration".

Different intravenous iron products are given over very different times and at different intervals, and the MHRA is explicit that "the prescribing, dosing, administration, and safety information differs between IV iron product formulations". This page does not publish doses or regimens; ask which product you are having and how long it takes when the appointment is booked.

Common effects afterwards include a metallic taste during the infusion, a headache, aching joints and temporary flushing. Brown staining of the skin at the cannula site can happen if the iron leaks out of the vein, and it can be long-lasting, so tell the nurse immediately if the site stings or swells.

What to expect afterwards

Haemoglobin does not jump overnight. A repeat blood test is usually arranged a few weeks later to check the response, and the underlying cause of the deficiency still needs addressing — NICE CKS emphasises that "the underlying cause of iron deficiency anaemia should be determined and treated" rather than treating the number alone.

Correcting anaemia before birth matters because it reduces how badly you tolerate normal blood loss at delivery. If you are close to term and severely anaemic, your team may also discuss birth planning and where you give birth. Continue any oral iron only if your team tells you to; taking both is not automatically better.