  The autonomic nervous system (ANS) regulatory capacity begins before birth as
the sympathetic and parasympathetic activity contributes significantly to the
fetus' development. Several studies have shown how vagus nerve is involved in
many vital processes during fetal, perinatal and postnatal life: from the
regulation of inflammation through the anti-inflammatory cholinergic pathway,
which may affect the functioning of each organ, to the production of hormones
involved in bioenergetic metabolism. In addition, the vagus nerve has been
recognized as the primary afferent pathway capable of transmitting information
to the brain from every organ of the body. Therefore, this hypothesis paper
aims to review the development of ANS during fetal and perinatal life, focusing
particularly on the vagus nerve, to identify possible "critical windows" that
could impact its maturation. These "critical windows" could help clinicians
know when to monitor fetuses to effectively assess the developmental status of
both ANS and specifically the vagus nerve. In addition, this paper will focus
on which factors (i.e. fetal characteristics and behaviors, maternal lifestyle
and pathologies, placental health and dysfunction, labor, incubator conditions,
and drug exposure) may have an impact on the development of the vagus during
the above-mentioned "critical window" and how. This analysis could help
clinicians and stakeholders define precise guidelines for improving the
management of fetuses and newborns, particularly to reduce the potential
adverse environmental impacts on ANS development that may lead to persistent
long-term consequences. Since the development of ANS and the vagus influence
have been shown to be reflected in cardiac variability, this paper will rely in
particular on studies using fetal heart rate variability (fHRV) to monitor the
continued growth and health of both animal and human fetuses.
