Melissa Joan Hart Melissa Joan Hart

Melissa Joan Hart has lost 40 pounds since joining Nutrisystem and now she's showing it off in a new commercial.

The Sabrina the Teenage Witch alum joined the program in February 2013 after giving birth to her third child, 17-month-old Tucker. Hart, who is also mom to Mason, 8, and Braydon, 5, said she gained 60 pounds during her third pregnancy but has since dropped three jean sizes thanks to Nutrisystem.

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"I wanted to look fantastic going into my 40s," Hart, clad in blue bathing suit, tells ET in a behind-the-scenes video from her commercial shoot. Check out her fit new physique in the video below: