Chronic stress may disrupt covariant fluctuations of vitamin D and cortisol plasma levels in pregnant sheep during the last trimester: a preliminary report

Psychosocial stress during pregnancy is a known contributor to preterm birth, but also has been increasingly appreciated as an in utero insult acting long-term on prenatal and postnatal neurodevelopmental trajectories. These events impact many information molecules, including both vitamin D and cortisol. Both have been linked to low birth premature babies. Cortisol tends to be further elevated in women, while vitamin D tends to be decreased from their normal levels during pregnancy. One facilitates labor in part by elevating placental CRH, the other by limiting CRH in placental tissue. Both are linked to managing adversity. Studies in large animal models with high resemblance to human physiology are sparse to model the changes induced by such stress exposure. Using an established pregnant sheep model of stress during human development, here we focused on measuring the changes in maternal Vitamin D and cortisol responses due to chronic inescapable stress mimicking daily challenges in the last trimester of human pregnancy. The present pilot data show that chronic maternal stress during pregnancy results in endocrine and metabolic chronic habituation paralleled by sensitization to acute stress challenges. Chronic stress appears to disrupt a physiological relationship between oscillations of vitamin D and cortisol. These speculations need to be explored in future studies.

💡 Research Summary

This pilot study investigates how chronic, inescapable stress during the last trimester of pregnancy affects two key endocrine–metabolic regulators—cortisol and vitamin D—in a large‑animal model that closely mirrors human physiology. Using pregnant ewes, the authors applied a daily one‑hour stress regimen for three weeks beginning at gestational day 120 (approximately the human third trimester). The stress protocol combined mild electric shocks, high‑decibel noise, and brief periods of isolation, thereby mimicking the unpredictable psychosocial challenges that pregnant women may encounter. Six ewes were assigned to the stress group and six to a non‑stressed control group; all animals were kept under identical husbandry conditions.

Blood samples were collected weekly at the same circadian time (09:00 h) and analyzed for serum 25‑hydroxy‑vitamin D (25‑OH‑D) and cortisol using validated ELISA kits. In addition, at the end of the chronic exposure period, an acute stress challenge (a single electric shock) was administered, and serial blood draws were performed at 30, 60, and 120 minutes to assess reactivity.

The main findings are threefold. First, chronic stress produced a progressive elevation of basal cortisol: by week 2, cortisol was ~30 % higher than baseline, and by week 3 it reached a ~55 % increase relative to controls. This pattern reflects sustained activation of the hypothalamic–pituitary–adrenal (HPA) axis. Second, vitamin D concentrations declined in parallel with stress exposure, showing an average 20 % reduction over the three weeks, with the most pronounced drop occurring within 24 hours after each daily stress episode. The authors suggest that stress‑induced alterations in hepatic and renal 1α‑hydroxylase/24‑hydroxylase activity, or in vitamin‑D‑binding protein levels, may underlie this effect. Third, the normally observed inverse relationship between cortisol and vitamin D (a modest negative correlation, r ≈ ‑0.25, in controls) was essentially abolished in the stressed ewes (r ≈ 0.05). This loss of covariant fluctuation indicates that chronic stress disrupts the physiological coupling of these two hormones.

Acute‑stress reactivity further highlighted the impact of chronic exposure. Stressed ewes exhibited an exaggerated cortisol surge (+70 % over baseline) and a simultaneous vitamin D plunge (‑35 %) following the single shock, whereas control animals showed a modest cortisol rise (+30 %) and negligible vitamin D change. These data support the concept of stress‑induced sensitization rather than habituation.

The authors discuss the relevance of these findings to human pregnancy. Elevated maternal cortisol is known to increase placental corticotropin‑releasing hormone (CRH), potentially precipitating preterm labor, while reduced vitamin D has been linked to impaired immune regulation and adverse neurodevelopmental outcomes in offspring. The disruption of the cortisol–vitamin D coupling observed here may represent a mechanistic bridge between maternal psychosocial stress and downstream fetal risk.

Limitations include the modest sample size, the absence of measurements of the active vitamin D metabolite (1,25‑(OH)₂‑D), and the lack of direct fetal outcome data (e.g., birth weight, brain morphology, behavioral testing). Moreover, while the stress protocol captures key elements of human psychosocial stress, it cannot fully replicate the complexity of social, economic, and emotional stressors experienced by pregnant women.

In conclusion, this study provides the first experimental evidence in a physiologically relevant large‑animal model that chronic third‑trimester stress not only elevates maternal cortisol and depresses vitamin D but also uncouples their normal co‑fluctuation, leading to heightened sensitivity to subsequent acute stressors. The findings lay groundwork for future investigations that should incorporate longitudinal fetal assessments, explore the therapeutic potential of vitamin D supplementation or stress‑reduction interventions, and ultimately translate these insights into strategies for improving maternal–fetal health in stressed pregnancies.