Queen Ethelburga and Bishop Paulinus, in York Minster(© Liz Coleman)
Æthelberht became the first Christian king in England following the arrival of the mission from Rome led by St Augustine in 597. He seems to have invited the mission under the influence of his queen, Bertha, Christian daughter of King Charibert I of the Franks and Queen Ingoberga. Bertha can have been born no later than about 562, since her parents separated in 561. At a time when girls were often married at age 14 or 15, and were bearing children from their mid teens onwards, it seems doubtful if Bertha could have married Æthelberht much later than around 580, as otherwise she would have been quite old by the standards of the time and this would not have been seen as an advantageous alliance from the point of view of the Kentish Kingdom. That Bertha was Æthelburh’s mother is nowhere stated explicitly, but it is implied by Bede in his Ecclesiastical History, completed in 733.
If Bertha was 18 in 580, normal human biology and simple arithmetic suggest that Æthelburh could have been born in the early 600s, though by then Bertha was probably at least 40, which at the time was a very late age to be having children. We know that Bertha died after 601, when the Pope sent her a letter. We know too that Æthelberht's widow married his son Eadbald after his death. It seems more likely that Eadbald married his step-mother rather than his biological mother, thus the conclusion is that Bertha died sometime before Æthelberht's death in 616, though we do not know when. Æthelberht’s second queen seems to have been a pagan, and it is perhaps unlikely that Æthelburh, who is known as a Christian princess, would have been born of a pagan mother. So if we accept that it is most likely Bertha was her mother, it seems likely too that she was born sometime between about 600 and perhaps 605, with a later date more probable because of her later life-story.