Leah Messer's 2-Year-Old Ali: Finally Walking!

Credit: Sony Pictures TV/Splash News

Leah Messer tied the knot with fiance Jeremy Calvert on April 4, but that's not the only happy news the 19-year-old has to celebrate.

The Teen Mom 2 star's developmentally delayed 2-year-old, Aliannah, "can walk when someone stands behind her, holding her hands," a source close to Ali's dad, Corey Simms, tells the new issue of Us Weekly. "It's a major step toward walking solo."

And a major milestone, given the fact that Messer and Simms -- who married in October 2010 and split six months later -- worried their daughter would have her share of physical limitations.

Aliannah is determined to catch up to her twin sister. "She wants to be able to run around an play with Aleeah!" says the source.

For more on Ali's progress -- and details on Messer's surprise wedding to Calvert, which took place with MTV cameras in tow -- pick up the new issue of Us Weekly, out now!