Oct. 10, 2011  -- Amber Miller accomplished two monumental feats this weekend.

Days from her due date, the 27-year-old joined 45,000 other runners to participate in Sunday's Bank of America Chicago Marathon and then gave birth to a baby girl named June hours later.

Miller, an avid runner, said she signed up for the 26.2-mile race before finding out she was pregnant. She said she never expected to finish the race.

"I was having a conversation with my parents and said, 'You know what? I have no plans of actually finishing,'" she told reporters at Central DuPage Hospital this morning. "I was planning on running half, skipping to the end, then walking across the finish line."

But Miller and her husband started running, and just kept going. They ran part of the race and walked the second half as her contractions started. It took the couple 6.5 hours to finish. She said she grabbed something to eat and the two headed to the hospital.

"It was very interesting hearing people's reaction," Miller said about crowds watching an extremely pregnant woman among the runners. "I've been running up to this point anyway, so I'm used to it."

At 7 pounds, 13 ounces, baby June entered the world at 10:29 p.m. Sunday, just hours after her parents crossed the finish line.

Marathon Mom: 'Outside of the Norm'

Dr. Jacques Moritz, a medical contributor for ABC News and director of gynecology at St. Luke's-Roosevelt Hospital, said that while Miller's story was "outside of the norm," he did not think she put her baby or herself in danger.

Moritz said new recommendations from the American College of Obstetricians and Gynecologists allowed a pregnant woman to do moderate, strenuous exercise as long as she could hold a conversation.

He said Miller, whom he has not treated, seemed to be in good cardiovascular health, was young and fit and ran throughout her pregnancy.

Moritz said that while she was not likely comfortable carrying the additional 20 pounds of baby during the race, "as long as she did not become winded during her exercise portion, the running part, she was fine."

"You have to be able to breathe," he advised pregnant women who wanted to exercise. "If you're not getting oxygen, the baby is not getting oxygen."

Miller has now completed eight marathons -- three of them while pregnant. She ran one when she was 17 weeks pregnant with June, and another when she was pregnant with June's older brother Caleb.

She said that it normally takes her about 3.5 hours to complete 26 miles. Miller said that after Sunday's grueling events, she just might hang up her sneakers for a while.