Pregnancy increases sympathetic nerve firing and decreases both basal parasympathetic activity and baroreflex functions. These changes are exaggerated in women with preeclampsia. The increase in sympathetic tone may counteract, to some degree, the profound vasodilation that is a hallmark of normal pregnancy. In contrast, the impaired baroreflex function is not beneficial for the mother. Angiotensin II via brain AT1 receptors may contribute to the marked increase in sympathetic nerve activity that begins early in normal pregnancy. The decrease in baroreflex function induced by normal pregnancy is mediated in part by increased levels of 3 alpha-hydroxy-dihydroprogesterone that enhances GABAergic suppression of the rostral ventrolateral medulla (RVLM) presympathetic neurons. In addition, reduced gain of baroreflex control of heart rate may be secondary to reduced actions of insulin in the arcuate nucleus via a neural pathway that includes the paraventricular nucleus of the hypothalamus and the RVLM.
The physiological adaptations to pregnancy are profoundly complex yet exquisitely tuned to ensure adequate development of the fetus. These changes are largely orchestrated directly or indirectly by the placenta, which is a source of a broad array of hormones, including neuropeptides, steroid hormones, pituitary-like hormones, growth factors, bioactive hormones, and metabolic hormones. The cardiovascular system and its autonomic innervation are particularly impacted. An early event in gestation is systemic vasodilation, which, at least in rats, is mediated in part by large increases in the corpus luteal hormone relaxin. Thus, the vasodilation begin during the luteal phase of the menstrual cycle, even before uteroplacental vascular growth and blood flow are significantly increased. As a result, arterial pressure falls. However, the decrease is modest because of parallel increases in cardiac output due in part to cardiac enlargement and increases in heart rate, but also to simultaneous activation of fluid-retaining factors (such as the renin-angiotensin-aldosterone system), which leads to increases in blood volume and cardiac preload.
In parallel to the increases in fluid-retaining hormones, sympathetic nerve activity gradually increases to reach a peak just before delivery. Using microneurography in humans, several studies have documented the rise in muscle sympathetic nerve activity, which begins early in gestation. In experimental animals, elevation in basal cardiac, lumbar, splanchnic, and, in some studies, renal sympathetic nerve activity has been observed. In line with these direct measurements of sympathetic nerve firing rates, indirect assessments, including quantification of heart rate and arterial pressure variability in both the frequency and time domains, also suggest that pregnancy increases basal sympathetic activity. Activation of the sympathetic nervous system would be expected to cause vasoconstriction, which could nullify the primary decrease in systemic vascular resistance. However, this effect is minimized during pregnancy because the vasculature becomes resistant to the actions of vasoconstriction, including norepinephrine. In parallel to the increases in cardiac sympathetic activity, cardiac parasympathetic tone decreases due in part to a reduction in the responsiveness of the heart to acetylcholine. One consequence of increased cardiac sympathetic activity and decreased parasympathetic activity is increased heart rate, which is also evident early in gestation.
The mechanism by which pregnancy increases sympathetic nerve activity has only recently been explored, with a focus on the roles of various circulating hormones and several brain sites. In brief, while an increase in the progesterone (sympathoexcitatory) to estrogen (sympathoinhibitory) ratio may explain the midluteal anticipatory increase in muscle sympathetic nerve activity in women. It is unlikely that these gonadal steroids are prominently involved during pregnancy, despite considerable rises in plasma levels. Similarly, while the levels of metabolic hormone partners, insulin and leptin, also increase, pregnancy renders their sympathoexcited reactions mute. On the other hand, several lines of evidence directly and indirectly support a role of angiotensin II. (Ang II) in brain
Pregnancy activates the systemic renin-angiotensin system in parallel to the gradual increases in SNA
Increases the expression of angiotensin II AT1 receptors (AT1Rcp) in brain
The pressor and sympathoexcitatory response to central angiotensin II Administration are potentiated during pregnancy.
Hypothalamic brain regions responsive to the sympathoexcitatory effects of Angiotensin II, including the subfornical organ, the paraventricular nucleus and the arcuate nucleus all support the elevated level of resting SNA during pregnancy.
Acute blockade of central and systemic AT1Rcp decreases renal SNA in conscious pregnant rabbits.
Nevertheless, experiments definitively establishing a role for AT1Rcp activation in select brain sites during pregnancy is lacking and requires further experimentation. Moreover, pregnancy induces sympathoexcitation to a greater extent than in many other physiological or pathophysiological states with elevated AngII Levels, suggesting that other unknown factors contribute. These might include inflammation and endothelin; however, again more work is needed.
While most physiological changes induced by pregnancy are relatively benign, one deleterious consequence is a marked suppression of the function of the baroreceptor reflex. As a result, pregnant women are prone to orthostatic hypotension, and pregnant animals are less able to maintain cardiac output, systemic vascular resistance, and arterial pressure during hemorrhage. Given that hemorrhage accompanies every delivery and that the baroreflex is largely responsible for arterial pressure maintenance during hemorrhage, it is understandable why peripartum hemorrhage is a major cause of maternal mortality.
Baroreflex dysfunction has been documented in several species besides humans, including rabbits, rats, goats, sheep, and dogs. Pregnancy attenuates baroreflex control of multiple efferents, such as SNA and heart rate, as well as hormones like vasopressin and ACTH. The decrease in baroreflex control of the autonomic nervous system has been detected using several methodological approaches, including noninvasive techniques in women employing the analysis of heart rate. And arterial pressure variability assessments of spontaneous baroreflex sensitivity and infusion of active drugs to construct complete sigmoidal baroreflex relationships between arterial pressure and HR or SNA. Three features of the HR sigmoidal baroreflex curve are attenuated: The maximum gain or slope of the most linear segment of the curve, the maximal level of HR achieved during severe hypotension. and the set point of the arterial pressure level associated with the midpoint of the curve (BP50). Pharmacological blockade of cardiac sympathetic or parasympathetic innervation has revealed that the sympathetic contribution to baroreflex control of HR is particularly depressed during pregnancy. Nevertheless, the spontaneous baroreflex sensitivity approach, which largely assesses the vagal efferent component of gain, is also depressed, indicating a reduced parasympathetic contribution as well. Baroreflex control of lumbar, splanchnic, renal, and, in women, muscle SNA has also been observed.
1. What normally happens postpartum
Pregnancy resets autonomic set points. Sympathetic firing rises (muscle sympathetic nerve activity is roughly doubled in late gestation), basal vagal tone falls, and baroreflex sensitivity is attenuated, partly because of volume expansion and vasodilation (nitric oxide, progesterone, relaxin). Delivery then removes the placenta abruptly, and volume shifts, hormones, and sleep deprivation all change at once.
In uncomplicated pregnancy, most measures normalize over weeks to a few months. Heart rate variability (HRV) and baroreflex sensitivity generally trend back toward prepregnancy values by roughly 6 to 12 weeks, though data are limited and heterogeneous. Lactation, anemia, and sleep loss prolong the transition. The analogy is a thermostat recalibrated for a new building: after you leave the building, the thermostat does not instantly return to its old setting, and a faulty thermostat will show its fault more clearly during the transition.
2. Why her course may differ
Gestational hypertension (preeclampsia spectrum): Sympathetic overactivity and impaired baroreflex function are more pronounced and may persist. Preeclampsia is associated with endothelial dysfunction and a higher long-term risk of hypertension and cardiovascular disease, and some cohorts show reduced HRV and baroreflex sensitivity months to years postpartum. Whether the autonomic abnormality is a cause or a consequence of the hypertensive disorder is unresolved.
Postpartum hemorrhage (PPH): Hemorrhage is a stress test of a baroreflex that is already attenuated and recovering. Severe hypovolemia drives intense sympathetic activation, and a blunted reflex means greater reliance on heart rate and catecholamines for pressure defense. Acute volume loss also produces iron deficiency and anemia, which independently cause tachycardia and palpitations and worsen orthostatic intolerance. Ferritin and hemoglobin must be checked before attributing symptoms to the ANS. Rarely, severe PPH leads to Sheehan syndrome (pituitary insufficiency), which can present with fatigue, hypotension, and palpitations. Screening is low cost if clinical features fit (failure of lactation, amenorrhea, fatigue).
hEDS substrate: Joint hypermobility with a family history of hEDS fits the recognized association with POTS, orthostatic intolerance, and other dysautonomia. Proposed mechanisms include excessive venous compliance and pooling, reduced vascular tone from connective tissue laxity, and a hyperadrenergic compensatory response. Pregnancy and delivery plausibly decompensate a borderline system. Note, however, that the strength of evidence for a causal link is moderate at best, and “hEDS” itself should be confirmed against the 2017 criteria in the patient and not assumed from family history.
3. A caveat on the physiology you cite
Attributing her palpitations to impaired baroreflex recovery is a reasonable hypothesis, but it is not established for this individual. Spontaneous, untriggered palpitations are nonspecific and have a differential that should be closed out first:
Postural tachycardia (POTS) vs. inappropriate sinus tachycardia vs. hyperadrenergic state
Anemia/iron deficiency, thyroid dysfunction (postpartum thyroiditis is common, typically 1 to 6 months postpartum, and may have a hyperthyroid phase)
Arrhythmia: PACs/PVCs, supraventricular tachycardia, atrial fibrillation (ambulatory monitor with symptom correlation)
Peripartum cardiomyopathy (echocardiogram if any dyspnea, edema, or reduced exercise tolerance; the hypertensive disorder raises risk)
Mitral valve prolapse, which is more common in connective tissue disorders
Residual hypertension, caffeine, medications, anxiety, sleep deprivation
Pheochromocytoma is rare but consider if paroxysmal with hypertension, headache, and diaphoresis
4. Suggested autonomic and ancillary workup
Active stand or 10-minute NASA lean test with HR and BP at 1, 3, 5, 10 minutes: orthostatic HR increment of 30 bpm or more (40 or more under age 19) without orthostatic hypotension supports POTS.
Formal autonomic reflex screen (QSART, heart rate response to deep breathing, Valsalva maneuver with phase IV overshoot, tilt table), with attention to the Valsalva pattern as an index of baroreflex adrenergic function.
Supine and standing plasma norepinephrine if hyperadrenergic POTS is suspected.
CBC, ferritin, TSH, BMP, morning cortisol if pituitary insufficiency is a concern.
12-lead ECG, extended ambulatory rhythm monitoring, and echocardiogram.
Consider small fiber neuropathy evaluation if sensory or distal sudomotor features exist.
Consider COMPASS-31 as a baseline symptom metric.
5. Timing matters
If she is within roughly 3 to 6 months postpartum, some of this may still be physiologic recovery, and the workup should account for lactation, anemia, and thyroid shifts. If symptoms have persisted beyond 6 months, a durable autonomic disorder (POTS in the setting of hEDS) becomes the more probable explanation, and the pregnancy complications likely served as the precipitant rather than the sole cause. Postpartum and post-hemorrhage onset is a recognized trigger pattern for POTS, as are infections and surgery.
6. Management implications if POTS is confirmed
Non-pharmacologic measures first (fluid 2 to 3 L/day, sodium 3 to 10 g/day if no hypertension, compression garments, recumbent exercise program), treat iron deficiency, and then consider pharmacotherapy (low-dose beta blocker, ivabradine, midodrine, pyridostigmine, fludrocortisone) selected by phenotype. Her history of gestational hypertension warrants caution with salt loading and fludrocortisone, so check current blood pressure. Lactation status affects drug choice.
7. Evidence limits
Most postpartum autonomic data come from small studies with short follow-up, and the link between hEDS and autonomic dysfunction rests largely on observational and survey data. I can run a PubMed search to pull specific studies on postpartum baroreflex recovery and on hEDS-associated dysautonomia if you want citations for your notes.
According to PubMed, there are good data on pregnancy and preeclampsia autonomic physiology and postpartum recovery. I found no direct human data on postpartum hemorrhage and baroreflex recovery, and none on postpartum-onset POTS in hEDS.
1. Normal pregnancy: sympathetic activity and baroreflex
Greenwood et al. recorded single-unit muscle sympathetic nerve activity (MSNA) in normal pregnancy, gestational hypertension, and non-pregnant controls. Single-unit sympathetic discharge was about double the non-pregnant value in normal pregnancy (38 vs 19 impulses/100 beats) and far higher in gestational hypertension (146). Cardiac baroreflex sensitivity (BRS) was impaired in both pregnant groups. After delivery, sympathetic activity fell to non-pregnant values and BRS rose. This is your core physiology. (DOI)
Hissen et al. followed 24 normotensive women longitudinally. Sympathetic BRS at 6 to 10 weeks postpartum did not differ from 18 non-pregnant controls, so in uncomplicated pregnancy the postpartum state appears to reflect the non-pregnant baseline. (DOI)
D’Souza et al. found that late-pregnancy supine MSNA burst frequency was higher, but orthostatic increases in burst frequency and strength were attenuated. The authors read this as a protective adaptation against sympathetic hyper-reactivity. (DOI)
2. Hypertensive disorders: slower and incomplete recovery
Walther et al. studied women with preeclampsia and healthy controls before and after delivery. Four days postpartum, arterial stiffness markers had normalized, but none of the HRV or BRS parameters had. (DOI)
van Wingerden et al. studied 70 women within 7 days postpartum. Those with preeclampsia had lower high-frequency HRV, a higher LF/HF ratio, and lower time-domain HRV, with or without chronic hypertension. (DOI)
Eneroth et al. used 24-hour Holter monitoring at 3 to 6 months postpartum. The high-frequency component stayed reduced in women with prior preeclampsia, which the authors interpreted as impaired vagal modulation outside pregnancy. This is a small study (15 per group) from 1999. (DOI)
Usselman et al. studied women 13 ± 6 months after preeclampsia. MSNA was higher (22 vs 13 bursts/min), cardiovagal BRS was blunted (15 vs 28 ms/mmHg), and central arterial stiffness was elevated, compared with women with uncomplicated pregnancies. Blood pressure was similar between groups, which the authors attributed to compensation from preserved sympathetic vascular BRS. n = 12 per group. (DOI)
Yousif et al. reviewed 26 studies. Of 453 women with preeclampsia, 93.6% showed some autonomic abnormality. Reduced parasympathetic activity (76% of studies reporting it) and reduced baroreflex gain (4 of 5 studies) were the most consistent findings. Autonomic testing could not separate mild from severe disease. (DOI)
3. Gaps, and one point to reconsider
Hemorrhage: My search returned no human study of hemorrhage challenging a pregnancy-attenuated baroreflex. The only hit was a rat study of osmolality and baroreceptor firing (Hines et al., DOI), which does not address hemorrhage. The statement that the baroreflex is largely responsible for pressure maintenance during hemorrhage is physiologically reasonable but not something I could source here. It is an inference, not a finding. The same holds for linking hemorrhage to persistent autonomic injury in this patient.
Postpartum POTS and hEDS: I found nothing directly addressing postpartum onset. Recent hEDS and POTS items are weak support: a pediatric letter (DOI), a single case report of POTS with functional neurologic disorder and hEDS (DOI), and a familial Chiari survey in which POTS was reported in 24% of familial vs 13% of sporadic cases (DOI). Cross-sectional and self-reported data dominate.
Correction to my earlier reply: I implied recovery generally by 6 to 12 weeks. That holds for uncomplicated pregnancy (Hissen). After preeclampsia, the data show persistence at 3 to 6 months and at about 1 year. Your patient had gestational hypertension, not necessarily preeclampsia, so I cannot say which group she resembles. The hypertensive-pregnancy data predict incomplete recovery, but the evidence rests on small studies of postpartum autonomic function.
4. Practical implication
The literature supports a plausible mechanism: pre-existing autonomic vulnerability, a hypertensive pregnancy with exaggerated sympathetic and baroreflex changes, then hemorrhage as a stressor. It does not establish that mechanism for her. Objective autonomic testing (Valsalva, deep breathing, tilt) is the way to document whether cardiovagal BRS and sympathetic adrenergic function are abnormal now, rather than assuming persistence from group-level data.
I can also search for hEDS-specific autonomic physiology (for example MSNA or tilt studies in hEDS) or for postpartum POTS case series with different terms.
Sources: All citations above were retrieved from PubMed; DOI links are provided with each study.