Internet - When he discovered the name his girlfriend picked out for their newborn, this man freaked out.

The new dad took to Reddit to share the story that has users in a spin.

He first met and fell in love with his current girlfriend in 2016 and the two now share a son.

When they started dating, she was apparently still getting over her ex, with whom she had been in a relationship for eight years.

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Although she was still heartbroken and hung up on the man who dumped her, her new partner's fears were eventually eased, especially when the ex moved back to their home city and they were introduced.

"I assumed she was totally over him," the original poster explained.

Four years later, his girlfriend got pregnant, despite being on birth control. While certainly unexpected, the couple was looking forward to rising their first child together.

Two months before the expected due date, the parents-to-be agreed on a name for their son. But when the man eventually looked at the baby's birth certificate, he was shocked to see it wasn't the name he had expected.

Written on the birth certificate was the name of the ex-boyfriend! Not only would their child have her ex's first name, but also his middle name.

When he confronted his girlfriend, she said it was a "spur of the moment thing" and avoided the discussion.

"She's been fobbing me off for a few days now so in the end I told her, calmly, that I'm not signing the birth certificate until we change the name back to what we agreed," he wrote in his post.

In the meantime, the poor man revealed he was so shaken by the event that he doubted both his paternity and his girlfriend's feelings for him.

Other users chimed in and advised him to get a DNA test as soon as possible.

"This is such a red flag," one of them commented.