The 'Dancing With the Stars' performer shared a video clip of himself pouring ice cold water over the pregnant actress' head on Instagram, with the caption: ''Hayden shortly went into labour after this... Jus kiddin (sic).''
The 'Nashville' actress also finally confirmed she is expecting her first child with fiancé Wladimir Klitschko before taking part in the challenge to raise money and awareness for Lou Gehrig's disease, which affects the nerve cells.
In the video, the 24-year-old actress gestures towards her large baby bump and says: ''I accept my ALS Challenge. I renominate Kacey Musgraves -- you're not getting out of this, Wladimir Klitschko, and my unborn child.''
After Derek, who is guesting starring on the upcoming season of the country music series, dumps the icy water over her head, the blonde beauty shouts: ''Holy shhhhhhhh. Yup, that's cold.''
News that Hayden and her Ukrainian boxer beau, 38, are to become parents first broke in May, and while the blonde beauty has stayed quiet about her pregnancy until now, Wladimir recently gushed: ''She's an amazing person and I'm more than sure that she's going to be an amazing mom.''