I was pleased to share this study with you:
https://www.aphis.usda.gov/animal_welfare/downloads/dangerous-animals/ACAids_Canine-MaternalStress_AC-19-005_6.19.pdf
stress triggers the release of hormones such as cortisol into the bloodstream. when these hormones cross the placenta from the mother to the puppies, the set point of the puppies' stress response system changes.
Stress also alters the quality of maternal care, which affects puppies' behavior.