Bertha's marriage to the king in 1072 was a result of peace negotiations between him and her stepfather, Count Robert I of Flanders.

That marriage was a scandal since both Philip and Bertrade were already married to other people, at least until Queen Bertha died the next year.

Her father ruled a territory vaguely described as "Friesland west of the Vlie", which is where Bertha spent her childhood.

[2] In 1070, Robert the Frisian became involved in a war with King Philip I of France over succession to the County of Flanders.

[2][3] Bertha was, at the time, the lowest-ranking woman to marry a French king; no suitable princess could be found, since they were all too closely related to Philip for the marriage to any of them to be seen as perfectly valid by the Church.

Bertha had no kings among her traceable ancestors and lacked even tenuous links with the Carolingian that her predecessors could claim.

[5] The regal title she gained by this marriage was prestigious, but had little meaning, as she was confined to her husband's small royal domain that covered little more than areas around Paris and Orléans.

Saint Arnulf of Soissons warned her that doing so would incur the wrath of God and lead to her being driven out of the kingdom into exile, where she would die despised and miserable.

According to the English historian William of Malmesbury, Philip complained that Bertha was "too fat", though he was himself becoming too obese to ride a horse.