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ObjectiveTo estimate normal ranges for maternal vital signs throughout pregnancy, which have not been well defined in a large contemporary population.MethodsWe conducted a three-center, prospective, longitudinal cohort study in the United Kingdom from August 2012 to September 2017. We recruited women at less than 20 weeks of gestation without significant comorbidities with accurately dated singleton pregnancies. We measured participants' blood pressure (BP), heart rate, respiratory rate, oxygen saturation and temperature following standardized operating procedures at 4-6 weekly intervals throughout pregnancy.ResultsWe screened 4,279 pregnant women, 1,041 met eligibility criteria and chose to take part. Systolic and diastolic BP decreased slightly from 12 weeks of gestation: median or 50th centile (3rd-97th centile) 114 (95-138); 70 (56-87) mm Hg to reach minimums of 113 (95-136); 69 (55-86) mm Hg at 18.6 and 19.2 weeks of gestation, respectively, a change (95% CI) of -1.0 (-2 to 0); -1 (-2 to -1) mm Hg. Systolic and diastolic BP then rose to a maximum median (3rd-97th centile) of 121 (102-144); 78 (62-95) mm Hg at 40 weeks of gestation, a difference (95% CI) of 7 (6-9) and9 (8-10) mm Hg, respectively. The median (3rd-97th centile) heart rate was lowest at 12 weeks of gestation: 82 (63-105) beats per minute (bpm), rising progressively to a maximum of 91 (68-115) bpm at 34.1 weeks. SpO2 decreased from 12 weeks of gestation: median (3-97 centile) 98% (94-99%) to 97% (93-99%) at 40 weeks. The median (3-97 centile) respiratory rate at 12 weeks of gestation was 15 (9-22), which did not change with gestation. The median (3-97 centile) temperature at 12 weeks of gestation was 36.7 (35.6-37.5)°C, decreasing to a minimum of 36.5 (35.3-37.3)°C at 33.4 weeks.ConclusionWe present widely relevant, gestation-specific reference ranges for detecting abnormal BP, heart rate, respiratory rate, oxygen saturation and temperature during pregnancy. Our findings refute the existence of a clinically significant BP drop from 12 weeks of gestation.Clinical trial registrationISRCTN, ISRCTN10838017.

Original publication




Journal article


Obstetrics and gynecology

Publication Date





653 - 664


Nuffield Department of Clinical Neurosciences, Nuffield Department of Women's & Reproductive Health, the Department of Engineering Science, and the Centre for Statistics in Medicine, University of Oxford, Oxford, Newcastle upon Tyne Hospitals NHS Foundation Trust, Newcastle, Guy's and St Thomas' NHS Foundation Trust, London, the Department of Women and Children's Health, King's College, London, and the Oxford National Institute for Health Research Biomedical Research Centre, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom.


Humans, Longitudinal Studies, Prospective Studies, Pregnancy, Reference Values, Female, Vital Signs