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The hulking actor who played The Mountain on Game of Thrones revealed the happy news in a post on Facebook.
He said: "After an extremely hard 2 hours and 19 mins of pushing our healthy, strong, beautiful boy arrived weighing 3530 grams, 52 cm long.
"To finish the most beautiful, awe inspiring labour was the most wonderful phone call to my daughter to introduce her to her new little brother.
"We arrive back at home both mom and baby doing super well and spent our first night as three together.
"We have decided on a name and cannot wait to share it with you all soon."
Hafþór said that Kelsey remained calm when she realised her water had broken and there was definitely excitement in the air.
"I immediately jumped out of bed and said alright let's go, she stayed calm and replied with a laugh and said, no we are not going yet. I then asked to call our midwives to which she replied again no let's just wait for awhile," the 6'9" Icelandic actor wrote.
He was absolutely full of praise for his wife for getting through the childbirth without any serious issues.
"Throughout it all she did amazing," he said. "She was in complete control of her mind and body. She had strong opinions on exactly what she wanted and where she wanted to be.
"I am completely in awe of how strong she is, and so proud of her for everything she did to prepare herself throughout her pregnancy. It cannot be put into words how proud I am and how I feel about this experience."
Super excited for the happy couple and we can't wait to see this mini Mountain grow up to be like his dad.
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