As we said five years ago, it is unclear whether John, Marquess of Somerset and Dorset, really was the son of John of Gaunt or of Sir Hugh Swynford. Furthermore, the common law answer to that question may be different to the genetic answer, as we revealed that Swynford could well have died after the conception, or even the birth of John “Beaufort”.
As hathawaysofhaworth reminded us in a comment here, the mediaeval year commenced on Lady Day, 25 March. Thus January, February and most March dates fall later in the same year as do 25 March to December. This gives more scope for Swynford’s life to have overlapped with the life, or gestation (“pre-life”) of Catherine de Roet’s middle child of seven.
Whilst Somerset’s (half-?) brother, Henry Beaufort, was a Cardinal, he did have an illegitimate daughter, unlike Thomas, Duke of Exeter. As Jane married Sir Edward Stradling in about 1423, according to the Cardinal’s will, we are still faced with the strong possibility that all those still surnamed Beaufort after his 1447 death were descended from Sir Hugh Swynford.