Every year for five days, the men of Ulster were incapacitated by the same pain felt by women in labour. They were cursed in this way after they treated the figure of Macha, a goddess and mythological figure, unfairly at an assembly of Ulster men. In Irish mythology, Macha was the wife of Crunnchua.

At the assembly, Crunnchua witnessed a horse race. Those in attendance, including the king, declared that nobody could run as fast as the horses. Crunnchua boasted that his wife could beat the horses without a problem. The king took Crunnchua up on the challenge and demanded that Macha run a race, even though she was in labour. She begged for a delay until she had finished her labour, but the king refused. At the finish line, she gave birth to twins and her shriek of pain condemned the men of Ulster to a curse for nine generations.