The royal baby will have links on its mother's side not just to "commoners" but also to a royal dynasty from the middle ages. Channel 4 News takes a fresh look at the Middleton family tree.
When William and Kate tied the knot in 2011, much was made of the prince marrying a "commoner". True enough, the Duchess of Cambridge has coalminers and tradesmen dotted about her family tree. But look closer and the Middleton family crest has some clear flashes of blue blood too.
In fact it emerged there was a link to Prince William himself through Sir William Gascoigne - a Yorkshire knight who died in 1487 - which made the newly-weds very, very distant cousins, once removed.
With the Duchess of Cambridge about to give birth to a child who will become third in line to the throne, genealogy expert Patrick Cracroft-Brennan, from Cracroft's Peerage, has taken a fresh look at the family tree for Channel 4 News.
His latest research reveals another royal line through Kate.
He has identified a descent from Elizabeth Plantagenet, an illegitimate daughter of King Edward IV (pictured) by his mistress, Elizabeth Lucy (also known as Elizabeth Wayte).
In fact both Kate and William descend from Elizabeth Plantagenet's son, Roger Lumley, of County Durham. Their closest common ancestors are Sir William Blakiston, of Whickham, and his wife Jane Lambton.
This ups the game a little - making the pair 11th cousins once removed. William descends from Sir William through both his parents.
Even the greatest of families can decline over time. Patrick Cracroft-Brennan
Patrick Cracroft-Brennan says: "The duchess's royal ancestry comes to her through her ancestor, Sir Thomas Conyers, ninth baronett, of Horden, Co Durham.
"His father had been a glazier working in Chester-le-Street before he inherited the baronetcy from a cousin.
"There was no money or property to go with the title, and indeed Sir Thomas, as poor as the proverbial church mouse, spent several years living in the local workhouse.
"His three daughters all married local tradesmen, and his grand-daughter, the Duchess's ancestress, married a local coal miner. The Conyers were one of the great noble families in the north of England in the middle ages, and this sad story shows how even the greatest of families can decline over time."
So the new baby need not worry too much about those coalminers, then?
"Obviously through Prince William his/her roots are incredibly royal," Cracroft-Brennan said.
"The descent from King Edward IV through Kate adds slightly to this. There are many families [in the UK] who have descents from King Charles II's illegitimate offspring, from the Tudors and from the Plantagenets.
"The new baby shares not only Kate's ancestry but also that of Lady Diana Spencer, Prince Philip, the Queen Mother, Queen Mary and so on. Through them there are all sorts of 'unusual' connections: descents from Roman emperors, from Charlemagne, from King Harold as well as William the Conqueror, from El Cid, from the Kings of Portugal, Castile, Aragon, France, Ireland, Scotland, Hungary, Bohemia, Denmark, Norway, Sweden, Prussia, Italy, as well as a whole raft of English and Scottish peerage families."
To cap it off, Kate is also an eighth cousin seven times removed to George Washington, first president of the United States, and a 13th cousin twice removed to US wartime leader General George Patton.