The actress - who memorably played Gretchen in Mean Girls - announced the birth of her first child with husband Dave Neydar via Instagram this week.
Alongside a beautiful photo of the newborn, Lacey said: "And then my soul saw you and it kind of went, 'Oh, there you are. I've been looking for you.'" #TBT to the best day of my life. One week ago I became a mum to this precious angel. Julia Mimi Bella.
"Julia, you are my heart's deepest dream come true. I love you more than I could ever say. Welcome to this big beautiful world sweet girl!"
Lacey had been eagerly awaiting her baby's arrival, joking on social media last month that she felt like her pregnancy would never end.
"#47weekspregnant OK not really, but seriously, for real...am I going to be pregnant forever?" she asked. "Is this a thing that happens to some women?? This last week and a half or so is no joke! #pleasecomesoonlittlelady #readytopop."
Lacey, who also starred in TV series Party of Five, married husband David in 2013.