A little stress for a fetus goes a long way.

Many animals experience hypoxia – low oxygen levels – in their environment on a regular basis. Fish living in shallow coastal rockpools experience hypoxia on a daily basis, and humans can also feel the effects of hypoxia when hiking at altitude in thin air. Developing embryos also experience bouts of hypoxia, some due to the mother's uterus contracting, which can reduce blood oxygen by 25%. During hypoxia, norepinephrine (noradrenaline) is released into the bloodstream to protect us from the negative effects of hypoxia by increasing the heart rate and either relaxing or constricting blood vessels to ensure blood supply to crucial organs. Because hypoxia provokes norepinephrine release, and developing embryos experience hypoxia, Margie Ream and her colleagues from Duke and Tufts Universities wondered whether norepinephrine protects developing fetal mice from the damaging effects of hypoxia