Kim Guiley, a doula from California, already had a daughter, Hazel, and had chosen not to find out the sex of her baby after suffering a miscarriage during a previous pregnancy.
'After experiencing a loss, all that mattered to us was a healthy baby, and we would have been thrilled either way,' she explained to the Daily Mail.
'But over the course of my pregnancy, I had convinced myself we were having another girl.'
Images: Susannah Gill Photographic Storytelling
Photographer Susannah Gill was present during the birth of the family's second child, and managed to capture the exact moment when Kim 'opened the baby's legs to discover that we had welcomed a baby boy'.
'I was overcome with shock and joy, as you can see!' she exclaimed.
Part of the reaction stemmed from the fact that Kim had always dreamed of parenting a boy and a girl, as she'd grown up in this dynamic herself.
After discovering that her wish had come true, she managed to see the funny side of the photograph.
'Can't stop laughing at this awesome capture by our birth photographer @susannahgill_photostorytelling...,' she wrote, sharing the image to her own Instagram account.
'After a lifetime of dreaming about one of each, but months of feeling so strongly that we were having another girl, this is the moment we discovered Theo's sex!!'
Kim also uploaded a blog post containing the whole of Theo's birth story, and despite a difficult labour, she says she feels like the 'luckiest mama ever'.
'In that perfect moment, all of my dreams had come true,' she concluded. 'My family was complete.'