The 41-year-old actor proposed to Aniston, 43, on his birthday last month and said he wants to have children with her, reported Contactmusic.
"The night he proposed he told Jen that he wanted her to have his child. She's been joking that in a perfect world it will happen on the wedding night... In truth, she'll be overjoyed whenever it happens.
"It's no secret she wanted a baby yesterday, so they're not wasting any time. She's not in denial, she knows it may be difficult," a source said.
It was recently revealed the couple could tie the knot as soon as December and they are thinking of jetting to Kenya for a romantic blessing before having a more formal ceremony in Los Angeles when they return.
"Both Jen and Justin want a private and intimate wedding, and they love the idea of eloping and marrying abroad. Kenya will be an amazingly romantic break and the plan is to have some kind of informal blessing while out there. They may well undergo something more formal in LA immediately afterwards," the source added.