In his moment of disbelief after the pregnancy was revealed, Colby Grace even went as far as to ask his wife Ashley if she had cheated on him.
'I asked because it was such a moment of shock. But really there were no doubts,' he told The Mirror.
The couple already had four children - Lanie, 12, Noah, Colby's son from a previous relationship, who is eight, Kellan, five, and Paisley, three - but they're now a family of five after welcoming 'surprise' baby Sawyer in June.
'He was certainly a surprise,' Ashley laughed. 'I think Colby thought I was kidding. Then he asked if I'd cheated on him.
'I was surprised, as we have the best relationship ever. Colby said he never doubted that the baby was his, but there was something weighing on him.'
The pair, whom their friends have since dubbed 'the most fertile couple ever', decided to use the unexpected nature of the pregnancy in their hilarious announcement picture, which they hashtagged #vasectomyfail.
'When he got the vasectomy, people joked that it would fail. I didn't believe them,' Ashley said. 'But there was never any doubt in my mind we'd keep Sawyer.'
'He is our son. When he arrived, I couldn't believe it. I thought I was done with being a mum. He was always meant to be.'
However, while the Graces are thrilled to have Sawyer as a member of the family, Colby has since had another vasectomy to ensure that there's no more surprises in the pipeline.
'The surgery did it for free, Ashley explained. 'They were wonderful and so helpful.'
'After Sawyer, though, we aren't taking any more chances. Five is enough.'