Maternal hemodynamics in normal pregnancy: reference ranges and role of maternal characteristics

Objectives The main aim of this study was to construct reference ranges of maternal central hemodynamic parameters during pregnancy. The second aim was to determine the maternal and pregnancy characteristics that influence these hemodynamic parameters. Methods This was a prospective cohort study of low‐risk pregnant women attending for routine antenatal care at St George's Hospital, London, UK. Exclusion criteria included any medical disorder present at the time of study recruitment, or development of hypertension or intrauterine fetal growth restriction following study recruitment. Stroke volume (SV), cardiac output (CO) and systemic vascular resistance (SVR) were obtained using non‐invasive cardiac output monitoring (USCOM‐1A®). USCOM‐1A utilizes a non‐imaging probe in the suprasternal notch to obtain velocity‐time integrals of transaortic blood flow at the left ventricular outflow tract. Once the distribution of the data with respect to gestational age had been determined, maternal characteristics were added to the model to test whether they provided a significant improvement in the prediction of the median value. Results The study included 627 women with a singleton pregnancy. The estimated median CO was constant for a maternal age above 32 years, but was around 0.5 L/min higher for women aged ≤ 25 years (P < 0.001). Maternal weight (P < 0.001) and height (P < 0.001) significantly affected CO values and there was a significant interaction (P = 0.002) between them. In women with a height of less than 1.60 m, there was no association between median CO and weight; however, in those with a height exceeding 1.60 m, an increase in weight was associated with an increase in CO. SV was primarily associated with height (P < 0.001), although some positive association with weight (P < 0.001) could also be observed within the normal body‐mass‐index range. Greater height (P < 0.001) was associated with lower median values of SVR, with an estimated difference of around 120 dynes × s/cm5 between 1.60 m and 1.80 m. Advancing maternal age was associated with higher median SVR, with an estimated difference of around 50 dynes × s/cm5 between 25 and 35 years. Smokers had a lower SVR by 73.5 (95% CI, 8.6–138.4) dynes × s/cm5. Conclusion Maternal hemodynamics are influenced significantly by maternal age, height and weight. We provide USCOM‐1A‐specific reference ranges and a calculator for SV, CO and SVR in uncomplicated pregnancies that correct for maternal age, height and weight. This should enable clinical application and comparison in both uncomplicated and pathological pregnancies.