“Bortbyting”, or “Utbyting” in some regions of Sweden, is a Changeling. It was used in olden times to refer to a child that the superstitious people suspected had been swapped by an Elf or a Troll for a multitude of reasons. There were ways to detect Changelings, usually by strange behaviour, tricking them or if they had a tail (!) it was possible to spot them. It is now commonly used in a grimly jokingly manner to refer to accidental placements of infants in maternity wards or when a child is very dissimilar to its parents.
One story about a Changeling was that one mother suspected that her child was a Changeling and was prepared to destroy it by throwing it into an oven when an enchantress appeared. “Such a wicked thing you have planned to do to my child I would never have been able to do to yours!” and she swapped the children once more and disappeared.
My own theory as to the origins of this myth is that it was most likely more popular during the time when infanticide was not an uncommon practice, and it was meant as an excuse to get rid of children with sever mental disabilities like Down’s Syndrome. One descriptor is that Trolls live ten times as long as any human, and they age the same way. Meaning that a “troll child” would have the mental faculties of a four year old when it was actually 40.
Also fey folk and witches avoided baptised children, so it was a way for the church to encourage attendance.