MATERNAL haemoglobin concentrations showed a gestational age-dependent U-shaped association with preterm birth (PTB), with thresholds varying according to pregnancy stage and altitude, according to a multicentre cohort study involving 141,943 singleton pregnancies in China. The findings suggest that routine antenatal haemoglobin measurements may require more tailored interpretation to improve the assessment of PTB risk.
The retrospective cohort study analysed data from five medical centres in Yunnan, China, incorporating electronic medical records, preconception examinations, and geospatial information. It included singleton pregnancies delivered between January 2017 and July 2025.
Of the 141,943 singleton live births included, 12,607 (8.88%) were preterm. Researchers investigated associations between maternal haemoglobin concentrations and PTB across different gestational ages, with the aim of establishing gestation-specific and altitude-adapted thresholds.
The analysis identified a U-shaped association, indicating that both low and high haemoglobin concentrations were associated with increased PTB risk. However, the thresholds associated with this risk changed throughout pregnancy.
Low haemoglobin measured at 13 weeks (>13.50 g/dL) were also associated with PTB (AOR: 1.28; 95% CI: 1.04–1.55).
Altitude emerged as an important factor in interpreting maternal haemoglobin measurements. Thresholds associated with PTB were generally higher among women living at high altitude, while low haemoglobin concentrations from 21 weeks onwards showed a persistent association with increased PTB risk.
Further analyses examining PTB subtypes identified differences in the timing of these associations. Low haemoglobin concentrations were associated with medically indicated PTB earlier in pregnancy, while high concentrations during middle and late pregnancy were associated with both medically indicated and other PTB subtypes.
These findings highlight the importance of considering geographical and physiological variation when interpreting routine antenatal blood results.
The study suggests that applying a single haemoglobin threshold throughout pregnancy may not adequately capture PTB risk. Instead, assessment windows and thresholds may need to account for both gestational age and altitude.
For healthcare professionals, these findings could inform future approaches to antenatal monitoring and the identification of pregnancies at increased risk of preterm delivery.
However, the proposed thresholds require external validation before their wider clinical application. Further research will be important to establish whether gestational age-specific and altitude-adapted haemoglobin assessment can improve the accuracy of PTB risk stratification and support antenatal decision-making.
Reference
Yan G et al. Gestational age– and altitude-specific hemoglobin thresholds and candidate windows for preterm birth. JAMA Netw Open. 2026;9;(11):e2642037.
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