Muscle sympathetic nerve activity during pregnancy: A systematic review and meta- analysis

Pregnancy requires profound cardiovascular adaptations, including a significant increase in blood volume and peripheral vasodilation. A key regulatory mechanism for maintaining blood pressure during this period is the sympathetic nervous system. While researchers have long suspected that muscle sympathetic nerve activity (MSNA) increases during pregnancy, findings have historically been inconsistent. This systematic review and meta-analysis synthesized data from 27 studies (807 participants) to quantify these changes and assess how various pregnancy complications influence sympathetic nerve function.

The researchers found that healthy pregnancy is characterized by a marked increase in MSNA, both in terms of burst frequency and burst incidence, which is observable by the first trimester and does not significantly change across gestational age. This state of sympathoexcitation is likely a necessary physiological adaptation to maintain blood pressure despite systemic vasodilation.

When examining complicated pregnancies, the picture becomes more complex. Individuals with obesity, obstructive sleep apnea, and gestational hypertension show evidence of sympathetic hyperactivity beyond what is seen in healthy pregnancies. Interestingly, this study contradicts earlier, smaller reports by finding that preeclampsia is not consistently associated with higher resting MSNA compared to uncomplicated pregnancies.

Regarding reactivity to stress, the study found that pregnant individuals show a blunted sympathetic response to head-up tilt (suggesting altered baroreflex engagement) but an exaggerated sympathetic response to the cold pressor test, despite similar cardiovascular outcomes in both groups. These findings suggest that neurovascular transduction—the way nerves influence blood vessels—may be altered during gestation.

Understanding how the sympathetic nervous system behaves during pregnancy is vital for identifying women at high risk for complications like preeclampsia or gestational hypertension. By establishing normative benchmarks for MSNA, this research provides a foundation for future studies to determine whether sympathetic hyperactivity is a causal factor in these complications or merely a symptom. These insights could ultimately improve clinical monitoring and help identify preventative strategies for cardiovascular health during and after pregnancy.