Amber Portwood will be joined by a very special guest on tonight's new episode of the 6th season of Teen Mom OG. According to the latest sneak peek, shared by 2 Paragraphs on September 11, Portwood is treated to a surprise birthday party by her fiance, Matthew Baier, who secretly invited her former cell mate to the bash.

As Portwood and a friendpull into a crowded parking lot in her neighborhood during the sneak peek, the reality star immediately reveals she's caught on to the surprise as she figures out that everyone she loves is waiting for her inside.

However, when it comes to her guest of honor, Portwood wasn't in the know.

After greeting her daughter, Leah, whose father Gary Shirley is also in attendance, Amber Portwood sees her cousin and several other members of her family and many friends before Baier calls in her former prison cell mate, Nicole, who walks in with a dozen white roses. As Portwood fights back tears, she manages to ask her friend how she is and tells her she looks beautiful.

Amber Portwood served 17 months behind bars

As 2 Paragraphs revealed, Amber Portwood entered prison in June 2012 and was released from the facility in Indiana in November 2013 after serving 17 months of her 5-year prison sentence. Although Portwood had an opportunity to avoid jail time by completing a drug rehabilitation program after receiving a felony drug charge, she opted out of the program and after being charged with aprobation violation, she turned herself in.

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While Portwood was behind bars, her ex-fiance, Gary Shirley, took over as Leah's primary custodian and began caring for the child full-time. Since then, however, Portwood has reclaimed some of her rights to her daughter and continues to spend tons of time with her on the weekends and throughout the week when Leah isn't attending elementary school.

To see more of Amber Portwood and her exciting birthday celebration, don't miss tonight's new episode of Teen Mom OG.

The series airs on Monday nights at 9 p.m. on MTV.

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