"She was having bad cramps for the past two weeks, and then she miscarried," a source toldOK! Weekly.
Reports that the 19-year-old reality TV star got engaged to current fiance Jeremy Calvert over the holidays and was expecting her third baby first surfaced earlier this month. While the couple recently had a lot to celebrate, they're now mourning the loss of their child.
"It's a devastating experience, but all I can do is support Leah and take things day by day," Calvert told OK! "It's hard, but we will get through it."
Messer was previously married to Teen Mom 2 co-star Corey Simms, the father of her two-year-old twin girls Aliannah and Aleeah. She also faced complications during her initial pregnancy, and Aliannah currently struggles with health issues.
While it hasn't been shown onTeen Mom 2's second season -- which premiered on MTV last month with Messer and Simms still together -- yet, Messer filed for divorce in April 2011 less than six months after the couple tied the knot in an October wedding which aired on MTV in March 2011 during the show's first-season finale.
Messer told Us Weekly last month that she and Calvert were talking about getting engaged.