To our knowledge, this is the first study to empirically demonstrate that stress is significantly associated with reduced female fecundity as measured by a lower probability of conception for each day during the fertile window as women’s salivary alpha-amylase concentrations rise. Moreover, the reduction in fecundability was mediated via the SAM pathway rather than through the HPA axis as evidenced by the opposing directions in FORs and the day-specific probabilities of conception for salivary alpha-amylase and cortisol, respectively. Irrespective of the day or frequency of sexual intercourse during the fertile window, women with higher concentrations of alpha amylase were less likely to conceive than women with lower concentrations underscoring the importance of a statistical model that is biologically responsive to the timing of intercourse relative to the fertile window and inclusive of other relevant covariates. While the findings were significant for alpha-amylase and the daily-specific conception probabilities in the first cycle, the results based on all cycles per woman were not. This most likely reflects a loss in statistical power with the most fecund women contributing one cycle coupled with the variability associated with alpha-amylase while trying.
Our findings do not support an earlier study involving 13 women prospectively followed that reported no differences in urinary adrenaline, noradrenaline, or cortisol concentrations between conception and non-conception cycles (26
). Among nulliparous Chinese textile workers trying to conceive, perceived stress during the follicular phase was associated with dysmenorrhea, underscoring the importance of timing when assessing stress-related effects (27
). While we did not measure dysmenorrhea in our study, all saliva samples were collected during the follicular phase. A recent cohort study of women undergoing their first in vitro
fertilization/intra cytoplasmatic sperm injection cycle reported greater treatment success for women with lower urinary adrenaline concentrations at oocyte retrieval, and lower adrenaline and noradrenaline concentrations at embryo transfer in comparison to women with unsuccessful cycles (28
). In addition, stress reduction behavioral therapies have been shown to improve IVF outcomes (9
The mechanisms by which alpha-amylase may reduce fecundity are as yet unknown, but the reproductive tract has long been known to contain catecholamine receptors (30
), which may alter blood flow through the fallopian tubes and gamete transportation (31
). Alpha-amylase is the principal salivary protein whose secretion from the parotid gland is regulated by the SAM system in response to sympathetic stimuli (physical and/or emotional stressors) resulting in increased blood catecholamines. Since this biomarker is produced locally in the oral cavity, it is in relatively high concentrations compared to other salivary markers such as cortisol that are serum constituents produced elsewhere in the body and transported to saliva via ultrafiltration (32
). To this end, alpha-amylase may be a novel biomarker for assessing psychosocial stressors and reproductive endpoints as mediated via the sympathetic nervous system. The opposing effects for stress biomarkers and fecundity observed in our study underscores the importance of measuring multiple stress biomarkers of stress-related systems such as the HPA and SAM.
While a promising biomarker, several important methodologic issues await further study to help interpret the findings. Our cohort was constructed within the Oxford Conception Study, a three-arm randomized trial to assess the efficacy of fertility monitor in helping women conceive. Our Bayesian models incorporated a woman-level random effect term to accommodate unobserved heterogeneity. The findings need to be interpreted within the context of utilizing the LH surge as a proxy for ovulation. Given the variability associated with ovulation, we have no reason to believe that the variation is nonrandom in this study cohort. Also, method of saliva collection including volume available for analysis may impact the measured concentrations (18
). Timing of saliva collection is an important consideration in that stress biomarkers may be affected by circadian rhythms. Nater and colleagues (33
) observed 76 participants who contributed 857 alpha-amylase measurements during the course of the day and reported a marked diurnal profile that was unrelated to perceived stress. However, a 4% increase in alpha-amylase was observed for every one point increase in the perceived psychosocial stress scale completed by participants. The authors also found that mean salivary cortisol concentrations did not predict alpha-amylase concentrations. Given the prospective nature our study, any biases arising from the salivary methodology should be non-differential driving the effects toward the null.
In sum, our findings support a reduction in the day-specific probability of conception among women with higher salivary alpha-amylase concentrations in comparison to women with lower concentrations. Our data support clinical and public health messages aimed at helping couples relax and minimize stressors when attempting to achieve pregnancy. This message becomes even more important when considering the maternal-fetal unit, given longstanding concern that stressors during pregnancy adversely affect fetal and infant well-being (34