Physiologic cardiovascular, respiratory, and thermoregulatory adaptations during pregnancy cause predictable changes in vital signs across gestation. The following trimester-specific reference ranges are derived from Green et al., Obstet Gynecol 2020.
| Vital Sign | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| Systolic Blood Pressure (mmHg) | 90-120 | 94.8-137.6 | 95.6-136.4 | 101.6-143.5 |
| Diastolic Blood Pressure (mmHg) | 60-80 | 55.5-86.9 | 56.8-87.1 | 62.4-94.7 |
| Heart Rate (beats per min) | 60-100 | 63.1-105.2 | 67.4-112.5 | 64.5-113.8 |
| Respiratory Rate (breaths per min) | 12-20 | 8-24 | 8-24 | 8-24 |
| Oxygen Saturation (SpO₂) (%) | 95-100 | 94.3-99.4 | 92.9-99.3 | 93.4-98.5 |
| Temperature (oC) | 36.5-37.3 | 35.55-37.51 | 35.35-37.37 | 35.37-37.35 |
| Temperature (oF) | 97.8-99.1 | 95.99-99.52 | 95.63-99.27 | 95.67-99.23 |
Physiologic Changes Affecting Vital Signs
- Blood Pressure: systemic vascular resistance decreases early → mild BP decrease in 1st-2nd trimester, returns to baseline in 3rd.
- Heart Rate: increases 10-20 bpm from prepregnancy baseline due to ↑ stroke volume and ↑ cardiac output.
- Respiratory Rate: unchanged, but tidal volume ↑ and minute ventilation ↑ by 30-50%.
- Oxygen Saturation: usually maintained ≥95% despite physiologic respiratory alkalosis.
- Temperature: progesterone increases basal temperature slightly early in pregnancy.
References
- Green LJ, et al. Gestation-Specific Vital Sign Reference Ranges in Pregnancy. Obstet Gynecol. 2020;135(3):653-664. PMID: 32028507