How can you go from being a servant girl one day and the next day you're going off on raids? And we're supposed to buy that this chick is carry a sword that weighs more than her body weight and that she's able to defeat seasoned warriors with ease. Oh and she's pregnant? Ffs.
She got worked and beaten in her first big battle. And she was a servant, not a seasoned warrior shield maiden, but they are brought up with the readiness and basic necessity to fight for your own. She got more serious training for this briefly by professionals, but doubt it was the first time she was doing it to some degree. Most of them are farmers and regular folk, but also warrior folk who expect if its necessary, even the elderly and children to pick up weapons.
Yeah she just got pregnant, and it was to show how naive and immature Björn is basically, for not using his head properly.