It's been two days since Jon Hamm and Jessica Westfeldt announced the end of their relationship after 18 years together - now source are claiming that marriage and kids was the couple's deal breaker.

A source told Us Weekly that the two were indecisive about starting a family. Hamm and Westfeldt reportedly tried to work out their differences in order to keep their relationship, but "the split was imminent."

Sources told New York Daily News that Westfeldt, 45, wanted a wedding and a baby, but Hamm was content with their current relationship status. The two have reportedly been encouraged by family members to end their relationship because they had grown tired of hearing the two complain about their differences on settling down and starting a family. 

"They were definitely not on the same page," one of the insider said.

Another source said, "They would talk about [having kids], and he always said 'maybe,' but last year it became 'No, I don't want kids."

During a 2012 interview with the Guardian, Westfeldt explained why she wasn't thinking about marriage or children, saying, "In reality our lives - Jon's and mine - are so unconventional in every way, it's very difficult. I think kids should have something grounded. Jon and I are both children of divorce.

However, a source confirmed to "Entertainment Tonight" that the couple's surprising split wasn't actually a surprise. A source claimed the former couple had been broken up for a few months now, blaming the split on the 44-year-old "Mad Men" star's "party-heavy lifestyle." 

"Jon and Jennifer really fought hard to make their relationship work, Jennifer more so than Jon," the source told ET. "They were together for many years, but in the end, it was too much work. It's no secret that Jon likes to go out and leads a very party-heavy lifestyle. And Jennifer was just tired of that."

The source later added, "She wanted him to grow up ... and he never wanted to. She was tired of being his mother." 

Hamm and Westfeldt began dating in 1997 and starred together in the 2011 film "Friends with Kids." In a joint statement to PEOPLE, the couple said, "With great sadness, we have decided to separate, after 18 years of love and shared history. We will continue to be supportive of each other in every way possible moving forward."