The couple made the announcement on their social media pages Sunday, with both parents' posts reading, "New year, new baby! Baby Mamba on the way 2019."
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Vanessa added ... "Kobe and I are so happy to add another baby blessing to our family! Bianka will have a new baby sibling with and Natalia and Gianna are super excited to welcome another little baby sister to love."
As for Kobe, he wrote ... "Vanessa and I are beyond excited to announce that we are expecting another #mambacita to go along with Natalia, Gianna and Bianka #blessed #bryantbunch #daddysprincesses #love #2019."
Kobe and Vanessa already have three daughters together -- Natalia, Gianna and Bianka ... aged 15, 12 and 2, respectively. KB and VB have been married since 2001.