Trish Staine, an aspiring half-marathon runner, thought her unbearable back pain was cause by a two-hour training session.
The 33-year-old said she had no idea she was pregnant before the surprise birth on Monday, Stuff.co.nz reported.
The Duluth mother of three said she hadn't gained any weight or felt fetal movement in the months before. And besides, her husband had a vasectomy.
She and her husband, John, have a daughter, 7, and a son, 11. She's also stepmother to John's three boys, ages 17, 19 and 20.
Staine said she ran for about two hours Sunday in preparation for the Garry Bjorklund half-marathon on June 22.
Later in the evening she had a sore back and her pain got worse the next morning. When her pain became unbearable, her husband called an ambulance.
During the emergency room examination, medical staff detected a fetal heartbeat and she was whisked to the delivery room.
About like 5 minutes later, at 3.25pm on Monday, she gave birth to her daughter who weighed 6 pounds, 6 ounces (2.9 kilograms), and was 18.9 inches (48 centimetres) long.
The girl, who is born about 5 weeks early, has been named Mira - short for Miracle.