The actress – who plays Dr. April Kepner in the hit medical drama – welcomed second child, daughter Hannah Mali Rose, on Wednesday, 3 December.
Hannah weighed 6lbs 14oz.
A rep for Sarah and her university lecturer husband Peter Lanfer told People that the family were "thrilled" to meet baby Hannah, who arrived three-and-half weeks early.
Sarah and Peter are also parents to son Micah, two.
Sarah later took to Twitter to provide a detailed explanation for her tot's name, saying she'd always loved the name Hannah.
"It means 'grace' or 'favour' and Hannah was also a Biblical figure who had tremendous faith in God," said Sarah.
She added: "There is a beautiful red bird named the Mali Firefinch for our redheaded early bird. She came 3.5 weeks early with a full head of red hair.
"In Thailand, Mali is the word for the jasmine flower. The scent of the jasmine flower has the ability to fill a room with sweetness.
"A close variant in Hebrew is the word male'/maleah meaning full or fullness of God. Hannah was born on the cusp of the full moon.
"After several days of rain in California the moon was amazing when Peter was driving home to be with Micah the day after Hannah was born.
"We chose Rose because it is one of the few flowers that blooms in December and we liked the way the names sounded together.
"And there you have it! Hannah Mali Rose Lanfer!"
The couple announced Sarah's second pregnancy in July, with Sarah tweeting a picture of her blossoming baby bump alongside the caption: "Nope, that's not a fake belly."
Congrats to Sarah and Peter!
Stars who welcomed babies in 2014!
Copyright: Rex Features WENN/Matthew Taylor/Daniel Deme/Josiah True/Matthre Taylor/Joe
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