Duff is also mom to 6-year-old son Luca, from her previous marriage to Mike Comrie.
Revealing the baby was a girl, she wrote: 'Guess what guys!
We wish Hilary and Matthew all the best on their new journey! "She will be as lovely and sweet as her mother.@hilaryduff another incredible chapter begins".
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Duff and Koma first started dating back in January 2017, just two years after they collaborated for the very first time on a couple of tracks from her album, Breathe In.
During the fifth season premiere of Younger in New York City four days ago, Duff was less ostentatious, as she wore a loose black dress. I exist, you know?
She said: 'I mean, this is like the third time that we've dated and I think that what [Selena] says has so much merit to it. Timing is such a big deal. third time's a charm. "You're always on my side!' She's such a traitor".