At the end of pregnancy, levels of the hormone oxytocin increase to stimulate parturition, or childbirth. The stress hormone cortisol is also important for normal fetal development and, like oxytocin, cortisol increases at the end of pregnancy. This may help explain why chronic stress during pregnancy increases the risk of complications such as stillbirth.
A new study published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology examined this link in ewes. They discovered that ewes with elevated cortisol in late gestation had more stillbirths (40% of lambs) than those with normal cortisol levels (25% of lambs). In addition, roughly one-quarter of ewes with high cortisol gave birth to a lamb prematurely. In contrast, there were no preterm lambs for ewes with normal cortisol levels. In addition to impacting pregnancy duration and birth, the researchers found that even full-term lambs born to stressed ewes showed signs of altered heart function right before and after birth. During delivery, the fetus normally experiences low oxygen concentrations which cause a decline in heart rate. Maternal stress appears to lower heart rate even more during these periods of hypoxia. Their findings also suggest that maternal stress can impact the ability of the newborn heart to adapt to life outside the womb.
The findings from this study help explain how anxiety and stress, as well as Cushing’s syndrome (oversecretion of cortisol) are linked to an increased risk of stillbirths in humans as well (even after ruling out the effects of maternal smoking, alcohol consumption, BMI, diabetes, and heart disease).
M Li, CE Wood, M Keller-Wood. Chronic maternal hypercortisolemia models stress-induced adverse birth outcome and altered cardiac function in newborn lambs. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 323(2), R193-R203, 2022. https://doi.org/10.1152/ajpregu.00041.2022