In case you didn't hear the explosion, JESSICA SIMPSON gave birth yesterday.  She and fiancé ERIC JOHNSON now have a healthy . . . and heavy . . . baby girl.

They named her Maxwell Drew Johnson, the baby weighed in at 9 pounds, 13 ounces . . . which is just three ounces shy of 10 pounds.  

Eric and Jessica's parents, along with Jessica's sister ASHLEE, were spotted arriving at the hospital yesterday morning before the delivery.

"Maxwell" is traditionally a boy's name . . . but it's a last name in Eric's family, and it's also his middle name.  "Drew" is Jessica's mother's maiden name.

Jessica announced the birth on her website, saying, quote, "Eric and I are elated to announce the birth of our baby girl, Maxwell Drew Johnson.

"We are grateful for all the love, support and prayers we have received.  This has been the greatest experience of our lives!!"

There's still no word on a wedding date for Jessica and Eric.  They've been engaged since November of 2010.

TMZ says Jessica's birthing suite at Cedars-Sinai Hospital is costing her $3,784 a day.  They also posted some pictures of it.  You can check 'em out here.