Third-trimester anaemia carries the strongest association with preterm birth

Design

Systematic review and random-effects meta-analysis of observational studies

Population

60 study populations, 2,119,392 pregnancies

Primary outcome

Preterm birth

Effect

OR 1.28 (95% CI 1.20–1.36); third trimester OR 1.65 (1.42–1.91); I² 95.5%

A meta-analysis pooled 60 study populations from 45 papers, covering 2,119,392 pregnancies. Maternal anaemia was associated with preterm birth, pooled OR 1.28 (95% CI 1.20–1.36). The association was strongest when anaemia was measured in the third trimester (OR 1.65, 1.42–1.91) and was not significant in the second (OR 1.10, 0.99–1.22). Studies from Asia gave OR 1.30 (1.22–1.40).

The caveats are large. Heterogeneity was extreme (I² = 95.5%) and Egger's test suggested publication bias (p = 0.001). All the included studies were observational, so anaemia may be a marker of poverty, infection, poor nutrition or short pregnancy spacing rather than a cause in itself.

What it supports is not a new treatment but a check that is easy to miss: haemoglobin at booking does not tell you about anaemia at 30 weeks. In India, where the National Family Health Survey (NFHS-5) found more than half of pregnant women anaemic, a repeat third-trimester haemoglobin is cheap and the finding is common.

• Repeat haemoglobin in the third trimester, not only at booking
• Look for the cause — iron deficiency, haemoglobinopathy, folate or B12 deficiency, hookworm — before assuming it is dietary
• Check that oral iron is actually being taken; gastrointestinal side-effects commonly stop it
• Consider intravenous iron where oral iron has failed and time to delivery is short
• Record anaemia as a preterm-birth risk marker when planning the place of delivery

Why it matters

Anaemia that develops late in pregnancy is the version most strongly linked with preterm birth, and the booking test cannot see it.

Don't overread it

These were observational studies — they show an association, not that treating anaemia prevents preterm birth.

The statistics, in plain English

An odds ratio of 1.28 means the odds of preterm birth were about 28% higher with anaemia; it is a relative figure, so the absolute difference depends on the background preterm rate. The second-trimester interval (0.99–1.22) just crosses 1.0, so no association can be claimed there. An I² of 95% means the studies disagreed far more than chance explains, so the single pooled number hides very different results, and Egger's test hints that small positive studies are over-represented.

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