Amber Miller holds her baby, June, next to her husband Joe in Central DuPage Hospital in Chicago October 10, 2011. [Photo/Agencies]

CHICAGO - Amber Miller could be forgiven for taking a long rest after completing the Chicago Marathon on Sunday but rather than resting her aching limbs, the 27-year-old spent the evening giving birth to her second child.

Miller was 39 weeks pregnant when she started the 26.2-mile (42.195-km) race and went into labour shortly after finishing, a spokeswoman from Central DuPage Hospital said on Monday.

The Chicago native had completed marathons during her previous pregnancy but told reporters at the hospital that she had not expected to finish this year's race.

"I was kind of planning on running about half and maybe skipping to the end and walking across the finish line," Miller said in a clip on the ABCNews website.

She ended up alternating between running for two miles and walking for two miles throughout the race, Miller told the Chicago Tribune.

After the race, Miller and her husband Joe settled down to eat dinner before she started to feel contractions in the middle of the afternoon and the couple headed straight to hospital.

Miller gave birth to her second child, June, at 10:29 pm on Sunday, the spokeswoman said, adding that the baby and her mother were both healthy.

Doctors were kept informed of Miller's exercise plans during her pregnancy and had no objections to her running in the marathon, the spokeswoman said.