The Geordie star, who is rumoured to be pregnant with her first baby, can be seen gasping in amazement during the promo while looking at pictures of her ancestors.
Gazing at one well-aged black and white photo, Cheryl sighs: "Wow, he's so handsome." She is later seen saying: "I feel really, really proud of him."
It is thought Cheryl is talking about her great-great-great grandfather, Battle Of The Somme war hero Joseph Ridley. While digging up her family tree, it was discovered that Mr Ridley is also the grandfather of journalist Yvonne Ridley, who was held captive by the Taliban 16 years ago.
The two did not know they were related, albeit distantly.
Meanwhile, away from the TV cameras, Cheryl is thought to be expecting a baby with her One Direction star boyfriend Liam Payne, however the pair have remained consistently tight-lipped on the rumours.
Last week, Cheryl cleverly dodged potential questioning about pregnancy whispers during a live appearance on The One Show.
The star was interviewed about her work with the charity Childline and host Matt Baker probed: "Are you enjoying life at the moment? How are you? You good?" With her hands in her lap, Cheryl replied: "I'm great, really well." Quickly batting the question back, she then asked: "Yourself?"
Cheryl's boyfriend Liam has since appeared to have requested some privacy with the help of a cryptic Instagram post. The picture shows two road signs, one named 'Liam Lane' and the other 'Private'. Liam captioned the snap: "Stay in your own lane people."
The BBC are yet to confirm the start date for Who Do You Think You Are?, but it will also feature Danny Dyer and Amanda Holden. Watch the trailer here:
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