"We hold hands cuz we are besties," the 32-year-old new mom captioned the image, which shows her holding her son's little hand in her own.
Sigler and her baseball player fiancé, Washington Nationals minor leaguer Cutter Dykstra, welcomed baby Beau on Wednesday.
"Thank you for all the love!" Sigler tweeted to well-wishers on Thursday. "@CutterDykstra and I feel so blessed and appreciative of everyone's well wishes. So madly in love with our BOY!"
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