Before 1 April 2015, but after 1 July 2001, there were different rules than those that apply today.
Children with a Swedish mother
A child with a Swedish mother always became a Swedish citizen. It did not matter if the child was born in Sweden or abroad.
Children with a Swedish father and a non-Swedish mother
A child of a Swedish father and a non-Swedish mother always became a Swedish citizen if the child was born in Sweden. If their father was married to their mother, the child acquired Swedish citizenship at birth, regardless of where in the world the child was born.
Children of a non-Swedish mother who was the spouse, registered partner, or cohabitating partner of a Swedish woman
A child of a non-Swedish mother who was the spouse, registered partner, or cohabitating partner of a Swedish woman also became a Swedish citizen if the child was born in Sweden and an insemination had taken place with the Swedish woman’s consent according to the provisions of the Children and Parents Code.
Swedish citizenship through the parents’ marriage
When a non-Swedish woman had children with a Swedish man and the child was born abroad, the parents had to be married to each other in order for the child to receive the father’s Swedish citizenship.
If the parents were unwed at the time of the child’s birth but later married, the child then became a Swedish citizen (provided that the child was under the age of 18 and themselves unwed at the time of their parents’ marriage). The process by which such a child became a Swedish citizen is known as “legitimation”. It is the Swedish Tax Agency that registers the child as a Swedish citizen. The child was then considered to have acquired Swedish citizenship on the date their parents married.
The Swedish Tax Agency
External link.