The 'Juno' actress - who is married to Ben Affleck - gave birth to her third child Samuel in February, while her pal and husband Jim Toth welcomed son Tennessee into the world in September, and the two actresses been providing moral support for each other as they attempt to get back to their pre-pregnancy figures.
A source told America's Star magazine: ''Both of them have a serious sweet tooth. So whenever Reese wants to reach for a chocolate bar she calls Jen instead and vice versa.''
Reese and Jennifer live in the same Los Angeles neighbourhood and attend Pilates classes together.
Reese - who also has children Ava, 13, and Deacon, nine, from her marriage to Ryan Phillippe - recently admitted she had been finding it hard to shed her baby weight.
She said: ''I don't know if I'm bouncing back. I'm slowly crawling back. You know, all of your joints feel wobbly, and your head is sort of confused.
''You just have to be gentle and patient with yourself and just sort of, you know...slowly get back to your health.''