"Bryan, Abigail and I couldn't be happier to welcome little Maile Pearl as the newest addition to our family," Duckworth, 50, said in a statement.
The milestone by the Illinois Democrat makes her the 10th member of Congress to give birth while in office.
The other nine women were members of the House of Representatives when they had their babies.
Duckworth was a House member when she had her first child, in 2014.
A decorated Asian-American US Army veteran, Duckworth lost both her legs in 2004 when the helicopter that she was co-piloting was shot down by insurgents over Iraq.
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She entered the House in 2012 and then the Senate in 2016, where she is one of the 100-strong chamber's 23 women.
Duckworth has made advocating for working class families a key focal point of her service in Congress.
In her statement she described parenthood as an economic issue affecting women and men alike.
"As tough as juggling the demands of motherhood and being a senator can be, I'm hardly alone or unique as a working parent, and my children only make me more committed to doing my job and standing up for hardworking families everywhere," she said.
Congratulations poured in from fellow lawmakers, including from top House Democrat Nancy Pelosi.
"I'm sure she will grow up to be just as strong as her mother!" Pelosi tweeted.
