Jen already shares three children with Ben - Violet, 13, Seraphina, 9, and Samuel, 6.
OK!'s source adds that despite Jen only being a few weeks along, she has already given Ben the heads up.
"Ben congratulated Jen and wanted to show his support, especially after the way she stepped in to help during his fall from grace," a source says, explaining that Jen dutifully ushered Ben to rehab in August after a sobriety relapse.
"Jen and Ben have always reassured each other that they'll always be family for their kids' sake.
"Jen's relieved she feels it's better to have Ben on her side in this. And now she can move forward and feel truly excited about the future."
Jen has not responded to OK!'s report.