That would definitely rule out training for a marathon, not that Chase was contemplating it. "New moms are extremely tired, so the prospect of getting on a treadmill or even running outside at six weeks when your children, at least my children aren't sleeping through the night" seems unimaginable, said Chase, who also has a 3-year-old daughter.

Except for women who have had a Caesarean section, which takes longer to heal, most women are ready to resume prior activity levels by eight weeks post-baby, Peaceman said.

"It's good to get back to the level that you were at prior to pregnancy, so for elite athletes, there's no reason they can't resume training," he said.

Pregnancy causes physical changes that both help and hinder athletic performance. Increased blood flow could be beneficial. But the girth from a swelling belly can change a woman's gait and make her feel off balance.

Also, joints and ligaments become more lax, which can raise the risk for injury, Szymanski said. Some doctors, including the American College of Obstetricians and Gynecologists, discourage most women from bouncy or high-impact workouts during pregnancy and urge them to take it easy at first after giving birth because some body changes linger.

The group says some lower-impact activities are generally safe during pregnancy, even for beginners, including swimming and walking.

Marathon running is not listed. Admirable as it is, Radcliffe's triumph is not unprecedented. Other elite long-distance runners have won races soon after having babies, including Norway's Ingrid Kristiansen, who won the 1983 Houston marathon within five months of giving birth.

It still seems way out of reach for moms who are not athletes. Says Marsha Takeda Morrison, a Los Angeles blogger whose children are 9 and 11, "It's been 3,285 days since I last gave birth and I get winded just walking to the refrigerator."

Click here to view the exercises you can do for a smooth delivery