The gold medallist rower-turned-tv star and his wife Beverley Turner welcomed daughter Trixie on Tuesday at their London home.
Cracknell says, "I'm absolutely thrilled. Beverley is beautiful and very tired and our new daughter is just gorgeous."
The couple already has a son Croyde, seven, and a two-year-old daughter Kiki.
Cracknell's baby joy comes less than a year after he suffered severe head injuries when he was hit by a truck while filming a cycling programme for the Discovery Channel in July.
The gold medallist was filming a cycling programme for the Discovery Channel in the U.S. last July when he was knocked off his bike by a truck in Arizona.
He was treated for a fractured skull as well as bleeding and swelling on the brain, but made a good recovery.
Cracknell now suffers problems with concentration, memory and his temper, and reveals he has been advised not to be alone with his two young children, seven-year-old son Croyde and two-year-old daughter Kiki, in case he accidentally leaves them in harm's way.
He tells Britain's Radio Times, "I feel a bit useless. I can't help out."
Cracknell also admits his injuries have affected his relationship with his TV presenter wife Beverley Turner, who is currently pregnant with the couple's third child.
He adds, "I regret that the injury has had such an impact on everyone. Bev and I talk about nothing else, which takes the fun out of our relationship. It must be hard for her.
"I would say that if we didn't have kids we would have broken up. Aside from the fact that Bev is upset by the changes she sees in me, which I can't see in myself, my condition is constantly on our minds. You can never get away from it, and that's been really hard."
The rowing ace was filming a cycling programme for the Discovery Channel in the U.S. when he was hit by a truck in Arizona.
He has revealed the accident jolted his brain and he now suffers problems with concentration, memory and his temper - but Cracknell's recuperation was boosted with some good news when his wife Beverley Turner discovered she is pregnant.
The gold medal-winning sportsman tells Britain's Daily Telegraph, "When the truck's wing mirror hit me on the back of the head some of the impact was absorbed by my helmet... Although I was very disorientated and agitated for some time, just knowing I'd been hit by a truck made me feel lucky to be alive...
"There is a crucial window of three weeks from the accident to regaining consistent memory. Within that window a full recovery is on the cards; outside of that and there are no guarantees. I have 'islands of memory' but no consistency, so there are no guarantees. I can't drive and will have to be reassessed.
"I can't drink for between six months and a year as the neurons in my brain rejoin. I will never be able to work at the same pace as before or be able to grasp complex theories. Although to be honest I've always tended to steer clear of the latter."
But Cracknell is adamant he will make a full recovery to be at his wife's side when she gives birth.
He adds, "I'm so proud of her and our unborn child for the way they have looked after each other and I'm determined to make sure I make a full recovery to be there for (my family)."
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