27-year-old marathon runner Amber Miller was nearly 39 weeks pregnant when she completed the Chicago Marathon on Sunday, but instead of putting her feet up and having a good rest, she went straight to hospital to give birth: her contractions started within minutes of crossing the finishing line.

Miller, who lives in Westchester, a suburb of Chicago, described Sunday as “the longest day of my life”. On that day she not only completed the 26.2 mile (42.16km) course, half running and half walking, in 6h 25m, she was also delivered of a healthy 7.7lb (3.5kg) daughter June, thus giving her one-year-old son, Caleb, a baby sister.

While recovering in hospital on Monday, Miller told the Associated Press in an interview that she was “crazy about running”, and that doing the marathon that day “wasn’t anything out of the ordinary” for her as she had been running regularly up to that point anyway.

Miller has been running for over 10 years. She had already signed up for the Chicago Marathon, her eighth, when she discovered she was pregnant. On the advice of her doctor she half ran and half walked it, drank lots of fluids and ate along the way.

She said she heard people cheering her, “Go pregnant lady!” which was a nice surprise as she had been expecting to hear negative remarks.

Although unusual it is not unheard of, for women to run long distance while pregnant. Miller ran the Wisconsin marathon earlier this year while 17 weeks pregnant, and she also ran the Indianapolis marathon in 2009 while pregnant with her son.

The current women’s marathon world record holder, English-born Paula Radcliffe, trained while she was pregnant and went on to win the New York marathon in 2007 only 10 months after her baby was born. She won the Chicago marathon in 2002.

Although she took a season away from competition while pregnant, American long distance runner Kara Goucher kept up her training, running 80 miles a week at times.

The usual medical advice to women who want to continue running long distance while pregnant, is as long as you are fit and healthy and are already used to running long distance, then it should be ok. But as with all exercise during pregnancy, don’t try anything new or anything your body is not used to.

There is little if any research on running during pregnancy, perhaps because, as Dr Rod Jaques, who advised British Triathlon Team at the British Olympic Medical Centre for 17 years until he stepped down in 2007, said in an article published in Runner’s World in 2002, scientists would be hard pressed to find women willing to subject themselves to such experiments while pregnant.

Jaques, advises pregnant women to keep their heart rate at or below 140 bpm while running and to take plenty of fluids.

Other experts suggest avoid running in the heat, and never run to the point of exhaustion.

Another point to bear in mind is that pregnant women are at higher risk of injury because they have higher levels of relaxin, a hormone that relaxes joints and ligaments, so they should ease gently into a run and stretch properly afterwards.

Also, because the hips gradually get wider in pregnancy to prepare for birth, you have a higher chance of overpronating, where the feet roll excessively inwards. This can affect the biomechanics of the knee and hip, so it might also be a good idea to see a podiatrist and ask advice about fitting your shoes with extra cushioning or inserts to improve stability.

When asked if she had to do the day over, Miller said, “definitely again”.

Written by Catharine Paddock PhD