Brown went to Beverly hospital after suffering "severe abdominal pain", which she believed was caused by gall stones. But nurses quickly alerted her she was actually in labour.
She told ABC News: "To understand and take in that I was pregnant and was about to go into labour, it was very overwhelming."
Brown and her husband Jason have been married for 22 years, but have never tried to have children.
The new mother said her approach was "if it happens, it happens" but after two decades of not using contraception, she didn't get pregnant.
Brown told WCVB she had noticed "some things going on with her body" over the past few months, but being pregnant was the last thing she expected.
She experienced no morning sickness and believed her weight gain was a sign she was going through the menopause.
Now, the couple are proud parents to a healthy 8lb baby girl - Carolyn Rose, named after the couple's mothers.
They borrowed a buggy from the hospital to take their daughter home and WCVB reports friends and relatives were rushing around helping them get everything ready for their daughter.
When new dad Jason was asked whether Carolyn Rose will be their only child, he replied: "I will get the surgery before it even becomes a thought."
Brown has since been thanking her generous neighbours and friends for helping her prepare things she needed for the baby.
She has been uploading photos of Carolyn Rose on Facebook.
"A great big thank you to the dart community who rallied round for this sweet little princess. Dart shooters are loving and caring people. Thank you very much for your generous hearts," she wrote on 9 November.