The aim was to examine the relationship between maternal haemoglobin (Hb) concentrations and risk of preterm birth by secondary analysis of data from a randomised controlled trial. This analysis included 10,430 women who were at least 20 years old and no more than 20 weeks of gestation. Results revealed neither first- nor second-trimester Hb concentrations were associated with the risk of preterm births. However, the risk of preterm birth increased when the Hb level was low (<130 g/L) in the first but high (≥130 g/L) in the second trimester, regardless of supplement type (iron-containing: AOR: 2.26, 95% CI: 1.37–3.73; non-iron-containing: AOR: 2.16, 95% CI: 1.11–4.21). In conclusion, maternal Hb concentrations were not associated with the risk of preterm birth. A low-Hb level in the first trimester but coupled with a high Hb level in the second was associated with an elevated risk of preterm birth.Impact statementWhat is already known on this subject: The relationship between maternal Hb concentration and preterm birth remains inconclusive. Some studies have shown an association between a low- or a high-Hb level and an increased risk of preterm birth. Others have not found such an association. Yet others have shown a U-shaped relationship.What do the results of this study add: Overall, maternal Hb concentrations in first or second trimester were not statistically associated with the risk of preterm birth. However, women with a low Hb concentration in the first trimester together with a high Hb concentration in the second trimester had an increased risk of preterm birth, compared to women who had a higher Hb concentration in the first trimester that remained similar during the second trimester.What are the implications are of these findings for clinical practice and/or further research: Our finding helps identify mothers who are at risk of having a preterm delivery. Investigating the underlying clinical causes of the unfavourable change in Hb levels and close follow-up to these women may help improve birth outcomes. What is already known on this subject: The relationship between maternal Hb concentration and preterm birth remains inconclusive. Some studies have shown an association between a low- or a high-Hb level and an increased risk of preterm birth. Others have not found such an association. Yet others have shown a U-shaped relationship. What do the results of this study add: Overall, maternal Hb concentrations in first or second trimester were not statistically associated with the risk of preterm birth. However, women with a low Hb concentration in the first trimester together with a high Hb concentration in the second trimester had an increased risk of preterm birth, compared to women who had a higher Hb concentration in the first trimester that remained similar during the second trimester. What are the implications are of these findings for clinical practice and/or further research: Our finding helps identify mothers who are at risk of having a preterm delivery. Investigating the underlying clinical causes of the unfavourable change in Hb levels and close follow-up to these women may help improve birth outcomes.

The aim was to examine the relationship between maternal haemoglobin (Hb) concentrations and risk of preterm birth by secondary analysis of data from a randomised controlled trial. This analysis included 10,430 women who were at least 20 years old and no more than 20 weeks of gestation. Results revealed neither first- nor second-trimester Hb concentrations were associated with the risk of preterm births. However, the risk of preterm birth increased when the Hb level was low (<130 g/L) in the first but high (≥130 g/L) in the second trimester, regardless of supplement type (iron-containing: AOR: 2.26, 95% CI: 1.37–3.73; non-iron-containing: AOR: 2.16, 95% CI: 1.11–4.21). In conclusion, maternal Hb concentrations were not associated with the risk of preterm birth. A low-Hb level in the first trimester but coupled with a high Hb level in the second was associated with an elevated risk of preterm birth.Impact statementWhat is already known on this subject: The relationship between maternal Hb concentration and preterm birth remains inconclusive. Some studies have shown an association between a low- or a high-Hb level and an increased risk of preterm birth. Others have not found such an association. Yet others have shown a U-shaped relationship.What do the results of this study add: Overall, maternal Hb concentrations in first or second trimester were not statistically associated with the risk of preterm birth. However, women with a low Hb concentration in the first trimester together with a high Hb concentration in the second trimester had an increased risk of preterm birth, compared to women who had a higher Hb concentration in the first trimester that remained similar during the second trimester.What are the implications are of these findings for clinical practice and/or further research: Our finding helps identify mothers who are at risk of having a preterm delivery. Investigating the underlying clinical causes of the unfavourable change in Hb levels and close follow-up to these women may help improve birth outcomes. What is already known on this subject: The relationship between maternal Hb concentration and preterm birth remains inconclusive. Some studies have shown an association between a low- or a high-Hb level and an increased risk of preterm birth. Others have not found such an association. Yet others have shown a U-shaped relationship. What do the results of this study add: Overall, maternal Hb concentrations in first or second trimester were not statistically associated with the risk of preterm birth. However, women with a low Hb concentration in the first trimester together with a high Hb concentration in the second trimester had an increased risk of preterm birth, compared to women who had a higher Hb concentration in the first trimester that remained similar during the second trimester. What are the implications are of these findings for clinical practice and/or further research: Our finding helps identify mothers who are at risk of having a preterm delivery. Investigating the underlying clinical causes of the unfavourable change in Hb levels and close follow-up to these women may help improve birth outcomes.