This meta-analysis evaluating the relationship between prenatal stress and offspring glucocorticoids across vertebrates shows (i) a positive overall effect size, though with 95% credible intervals overlapping with 0, (ii) stronger effects for experimental studies (as opposed to observational ones) and those reporting glucocorticoid recovery (as opposed to baseline or peak reactivity), and (iii) little variation in effect sizes across species or life history characteristics. These findings indicate that offspring sensitivity to prenatal stress is found across vertebrates despite large differences in body size, maturation rate, and other aspects of life history, but that the strength of this association varies based on study design. It also suggests that animal models may be appropriate for studying DOHaD effects related to prenatal stress and their impacts on offspring stress physiology.

Even though prenatal stress was overall positively associated with later-life glucocorticoid physiology, the 95% credible intervals included 0. Given the large number of species, study designs, and the range of effect sizes (Fig. 1) - which introduce heterogeneity and thereby increase the confidence intervals of the overall effect size - this finding is somewhat expected. More research with comparable study designs is needed to more accurately evaluate the overall effect. Nonetheless, the vast majority of the posterior probability distribution indicated a positive overall effect (PP > 0 = 0.94).

By evaluating different aspects of study design we were able to identify study factors that appear to influence the effect size between prenatal stress exposure and offspring glucocorticoids. In particular, experimental designs elicited greater effect sizes than observational designs. Based on prior studies7,25,26,27,28, we would predict that organisms would adjust their physiology and behavior in response to maternal experience of chronic as opposed to more randomly dispersed, acute stressors, the latter of which might be less reliable indicators of environmental quality. It is therefore possible that experimental studies, which are purposefully designed to induce a strong impact on maternal and/or offspring physiology, tend to be severe enough that this signal is more reliably passed on to offspring. As an example, exogenous administration of glucocorticoids to eggs is unsurprisingly associated with increased glucocorticoid levels in offspring29,30,31. Observational studies of perceived stress, however, do not always measure prenatal glucocorticoid exposure directly32,33, and therefore inconsistent results may result from offspring in observational studies being exposed to lower glucocorticoids prenatally.

In contrast to baseline or peak reactivity, we found that examining offspring glucocorticoid recovery following stressor exposure elicited the greatest effect sizes. Several factors could be responsible for this finding. First, we identified inconsistency in the number and timing of recovery measurements reported across studies. For example, Montano et al.34 assessed corticosterone recovery to stress in mice at hours 4, 8, 12, 16 and 24, respectively, while Ping et al.33 assessed cortisol recovery of human infants at 20 and 45 minutes after a stressor, respectively. When multiple values were reported for recovery, we selected the time point with the greatest difference between cases and controls. Because of this, our methodology selected for recovery measures to have the largest possible effects. Given variation in the timing of glucocorticoid measurements across studies, it is possible that some studies simply did not detect the highest peak glucocorticoid response after a stressor with their sampling scheme, or - similarly - that some studies did not measure baseline appropriately. That said, the influence of such flaws on the magnitude of effect sizes should be minimized as each study compared a stressed group to a control group and assayed baseline and reactivity in both groups using the same approach.

Second, there may be functional reasons why glucocorticoid recovery is more strongly associated with prenatal stress exposure than other measures. Specifically, differences in cortisol recovery following stress exposure may reflect the maladaptive impacts of increased allostatic load35. Repeated or chronic activation of physiological systems and their attempt to maintain homeostasis can result in “wear and tear”, which may then be indexed by a prolonged response due to a delayed or inefficient return to baseline, i.e. recovery36.

As mammalian mothers are able to directly influence offspring biology across both gestation and lactation37, we predicted that maternal effects would be stronger in mammals. However, we found no significant differences between mammals and non-mammals. In addition, we might anticipate that organisms with slower life history strategies (as indicated by larger size and giving birth to fewer, higher quality offspring) would have stronger evidence for maternal effects than species characterized by faster life histories38. Surprisingly, none of the life history variables were unequivocally associated with effect sizes. This suggests that species with fast or slow life histories have a similar capacity for prenatal stress-induced maternal effects. These findings are therefore consistent with an ancient vertebrate origin of HPA-axis programming, and suggest that such effects may have been selected for to mediate intra-specific variability in life history strategy13. Given the similarities in programming capacity across vertebrates, this suggests that both mammalian and non-mammalian model organisms may be appropriate for developing a detailed understanding of prenatal programming of HPA-axis function in humans.

Studies that have directly compared glucocorticoid levels in fish39 and bird eggs40 have recorded substantial variation both within and between species. These results, like ours, suggest that there are examples of prenatal sensitivity to stress in a broad range of species, though they are only detected under certain conditions. In particular, previous studies have found that timing of breeding, laying order, offspring sex, life history stage at assessment, and types of treatment can all affect glucocorticoid programming effects39,40. Both the universality and specificity of this programming response further suggests a potential adaptive function of maternal stress induced impacts on offspring glucocorticoids.

Despite the strength of this analysis, principally the evaluation of the impacts of prenatal stress using state-of-the-art statistical methods that account for all non-independencies and moderating factors, there were several limitations. First, the wide range of study designs made it difficult to directly compare severity and timing of stressors across species. In order to combat this, we attempted to define our variables as broadly as we could to avoid misclassification (as described in Table 1), and we have supplied raw data in our supplementary materials to facilitate future analyses. Second, though there is good evidence that glucocorticoids induce similar phenotypes in non-mammalian species (for example, higher glucocorticoids in fish eggs are associated with smaller offspring size39 similar to humans and other mammals), there were relatively few studies in non-mammals included here, which diminished our ability to detect broad phylogenetic patterns. More research is needed, but the limited findings suggest that the low phylogenetic influence on effect sizes might hold up in larger comparative samples. Third, our study focused specifically on glucocorticoids. Epigenetic marks, transport proteins, and hormone receptors also respond to environmental variability and therefore could be influenced by prenatal stress and shape offspring development41,42. Therefore, it is possible that prenatal stress impacts HPA-axis function through these pathways instead of (or in addition to) solely the effects of prenatal stress on glucocorticoids17,43,44,45.

Species in which the maternal environment most accurately predicts offspring environment are expected to have the most consistent examples of maternal effects10. It is difficult to study environmental predictability across species; this could also account for the somewhat ambiguous results reported here and in another maternal-effects meta-analysis46. Indeed, environmental predictability may be better studied in comparisons of different populations of the same species, or studies of the same population across time, such as the hare study described above7.

The studies in this meta-analysis included stressors that varied widely in terms of timing, chronicity, severity, and ecological relevance. If prenatal stress-induced maternal effects are meant to be adaptive, then we would expect that stressors that are predictable or cyclical in nature would induce these effects more reliably than random, unpredictable stressors47. In ecological studies this is commonly assessed by studying predator density and resource availability4. In observational human studies, however, the stressors often included factors such as perceived racial discrimination48 or pregnancy-specific anxiety49. While potentially chronic in nature, the stressors studied in human research are nonetheless qualitatively different than those studied in ecological research, making the results somewhat difficult to compare. In order to resolve these issues it is important to have more human studies in response to a broader range of stressors, as well as in non-Western cultural contexts. Understanding how prenatal exposure to stressors associated with climate change, predation, or other extrinsic mortality risk factors, for example, could provide a more appropriate model for comparing human studies with ecological models in other species. While not assessing cortisol in pregnancy, anthropologists have studied pregnant women experiencing seasonal and chronic nutritional stress among pastoralists50 and hunter gatherers51, suggesting that such studies are potentially feasible.

In sum, we found a trend toward elevated glucocorticoids in offspring exposed to prenatal stress. These effects were strongest in experimental studies and those assessing glucocorticoid recovery following stress exposure. The lack of a strong phylogenetic signal, and the fact that life history variables were not clearly associated with the response, is consistent with the interpretation that this response is evolutionarily ancient and conserved across a broad range of species.