AFTER giving birth within days of each other, brand-new BFFs Kim Kardashian and Jessica Simpson are bonding over their new babies and post-pregnancy paunches.

Kim turned to Jessica for advice early on in her pregnancy when she realized she was quickly packing on pounds. By the time daughter North arrived on June 15, a source says the 32-year-old reality star had gained 100 pounds and was tipping the scales at more than 220.

“Fashion Star” Jessica could relate – she also blew up during her first pregnancy with daugh­ter Maxwell, but managed to drop 70 pounds after she signed a $4 million spokes­person contract with Weight Watchers.

Fortunately for Jessica, 33, she didn’t gain nearly as much weight the second time around with son Ace Knute, who arrived on June 30.

“Kim reached out to Jessica toward the end of her first tri­mester, just as her weight was exploding,” revealed a source. “Kim sympathized with Jess over all the nasty comments about how huge she got when she was pregnant with Maxie, and Kim thought Jess could help her avoid the same pitfalls.”

Kim also asked Jessica for help in coming up with a game plan to lose the weight after she gave birth – and Jess was happy to share what worked for her, says the source.

“Before long, they were talking on the phone several times a day, sharing their tales of woe about  their bodily aches and pains, hormonal surges and about how their men were adapting.”

Kim’s baby daddy is hip-hop mogul Kanye West, while Jes­sica’s mate is her fiance, former NFL player Eric Johnson.

The friendship between the women has grown even stron­ger in recent weeks. Since they live just a few miles apart, they’ve been able to get to­gether at each other’s homes. Jessica has also been growing close to the rest of Kim’s fa­mous family, including sister Kourtney, whose daughter, Penelope, is the same age as Jess’ girl.

And new moms Kim and Jessica will be spending even more time together, as they’ve turned to celebrity train­er Gunnar Peterson to come up with a made-for-two custom workout program they can follow. The friendship has been an ego-saver for Kim, says the source, and she’s grateful she has some­one who understands how hard it is to go from “hot” to “not.”

“Jessica knows what it’s like to get slammed over weight gains,” the source added. “She’s been counseling Kim to ignore the negativity and just focus on her precious baby.”