Saber's full name is Arturia Pendragon, the daughter of British King Uther Pendragon and Igraine, a former Duchess of Cornwall. During her nativity, her father realized that his subjects would never accept a female as their legitimate ruler and thus decided not to publicly announce Arturia’s birth or gender. She was then entrusted by Merlin to a loyal knight, Sir Ector, who raised Arturia as a surrogate son.
I always found this story to be so sad, seeing Saber shunned because she was born female. Thanks to Lirica for sending this!