Muscle sympathetic nerve activity peaks in the first trimester in healthy pregnancy: a longitudinal case study

This longitudinal case study investigated how muscle sympathetic nerve activity (MSNA) and baroreflex sensitivity (BRS) change throughout a healthy, normotensive pregnancy. While pregnancy involves complex hemodynamic shifts—such as increased blood volume and systemic vasodilation—the role of the sympathetic nervous system in managing these changes remains partially understood. By tracking one participant from the pre-conception phase through all three trimesters and into the post-partum period, the researchers aimed to provide a detailed temporal map of autonomic cardiovascular control.

The study utilized microneurography to record MSNA directly from the common peroneal nerve. The participant was assessed once pre-pregnancy, seven times during pregnancy (at weeks 6, 11, 17, 22, 25, 33, and 36), and twice post-partum. Researchers measured blood pressure, heart rate, and MSNA to calculate both cardiac baroreflex sensitivity (the heart's response to blood pressure changes) and sympathetic baroreflex sensitivity (how the nervous system modulates sympathetic nerve activity in response to pressure changes).

The researchers observed a primary peak in MSNA as early as 6 weeks into gestation. Although MSNA declined slightly after this initial spike, it remained elevated relative to pre-pregnancy levels throughout the entire pregnancy. A secondary, smaller peak in MSNA was noted in the third trimester, which the authors suggest may be partly driven by the natural increase in resting heart rate during late pregnancy. Furthermore, cardiac BRS was found to improve during the first trimester before gradually declining, while sympathetic BRS showed variability but generally remained higher than pre-pregnancy levels. These results suggest that the early surge in sympathetic outflow is not merely a compensatory response to reduced baroreflex function.