[i]Women from noble Houses in Westeros often take on their husband's surname - though this is not automatic and many do not. Essentially, if a noblewoman "marries up" into a more powerful noble family, she will tend to shift to using the name of her husband's House. Conversely, if a noblewoman marries into a less powerful noble House (i.e., if she is one of the younger daughters in the family, not able to secure as good of a match) then she will often retain the use of her original family name, to remind everyone of her social and political ties. If their two families are of roughly equal power - such as if they are both from Great Houses that rule entire kingdoms - then the situation is more variable, and is more dependent on personal choice.[/i]

FYI