Kyle Busch's Wife -- Insane Bikini Bod ... 1 Week After Giving Birth

Pregnancy does a body good ... REALLY GOOD ... just ask the wife of NASCAR stud Kyle Busch -- who posted an UNBELIEVABLE bikini pic just one week after giving birth.

Samantha Busch posted a pic showing off her incredibly tight bod -- 6 days after popping out the couple's son Brexton Busch.

But Busch doesn't chalk up her super quick turnaround to good genes -- she worked her ass off during her pregnancy to stay fit and healthy.

In fact, Samantha blogged about her workouts ... crediting "Andrea Orbeck Pregnancy Sculpt" and "Suzanne Bowen’s Slim & Toned Prenatal Barre Workout" for keeping her fit while she was with child.

As for Kyle, he was raving about Samantha over the weekend ... saying, "She’s a champ. Samantha is my champion. No matter how well or how successful I ever am in my career, she’s got the championship trophy already on her mantle."