Amber Miller found completing the Chicago Marathon on Sunday a laborious feat — that's because she went into labour shortly after crossing the finish line and gave birth to a daughter hours later.

From her hospital bed, Miller, of DuPage County in Illinois, told the Daily Herald she took on the 42.2-kilometre challenge because her doctor said she could run half the race and then walk the rest of the way.

Miller, who was 38 weeks and five days into her pregnancy, told the newspaper she felt "something" as she crossed the finish line in less than 6½ hours, at around 3:30 p.m. local time.

Of the 45,000 participants in this year's Chicago Marathon on Sunday, 35,600 finishes the race. (Nam Y. Huh/Associated Press)"A few minutes after I crossed through the finish chute, the contractions were coming a little stronger than I was used to," she told the Daily Herald on the phone from her hospital bed. "That’s when I figured out what was going on."

Miller said she would feel contractions while running during both her pregnancies.

But after the marathon Sunday, the contractions were more regular, so she stopped and grabbed a sandwich and then went to the hospital.

"It was the longest day of my life," she said.

Her daughter, June, was born after two hours and 18 minutes of labour, weighing 7 pounds, 13 ounces.

Defending champion Liliya Shobukhova of Russia captured the women's portion of the race featuring 45,000 runners in 2:18:20. Her time is the fastest for a woman in the world this year.