Fans of "Teen Mom 2" have watched Leah Messer's daughter Ali go from doctor's appointment to doctor's appointment for four seasons ... and now, we have a diagnosis.

Aliannah has been diagnosed with a rare form of muscular dystrophy, the reality TV star reveals in a new interview with In Touch.

"The disorder affects her respiratory system and progresses the older she gets," the 21-year-old explains. "The bigger she gets and the more weight she gains, the harder it’ll be for her muscles to carry the weight."

Messer also says that her family, which also includes Ali's twin sister Aleeah, husband Jeremy Calvert and Adalynn -- the couple's first child together -- will be moving into a more handicap-friendly home soon.

"We were scared because we didn’t know what her future was going to look like, but we always pushed for answers," adds Leah. "We never gave up, and we never will. We’ll do everything we can to make sure Ali has the life that she needs."

"Ali’s condition has brought us all extremely close together," she says. "We have to stay strong for Ali, and things are going good."

"Teen Mom 2" returns for its fifth season tonight on MTV. See how all the girls' children have grown since Season One in the gallery above!

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