But many of us don't know how to care for it, and put up with pain, discomfort, even leakage. Here's why it happens and what you can do about it before it derails your progress.

WHAT FRESH HELL IS THIS?

Liz B. wondered during a four-mile run on a chilly December night five months after giving birth. She was 2.7 miles into an out-and-back route when she realized she'd leaked a little urine. "It happened on and off the rest of the way home," says the 36-year-old from central Ohio, who prefers not to use her full name when talking about urinary incontinence, which affects 25 million adult Americans.

This was not how she had envisioned her postpartum life. Liz, a physical therapist who works with stroke, brain injury, and spinal cord injury patients, had been a runner for nearly her whole life. Her father, who just ran his 91st marathon at the age of 77, had taken her out for jogs when she was a toddler, and she was hooked on long distances ever since. Her idea of a "good time" was waking up at 5 a.m. on a Saturday to go for a scenic 16-mile run with her sister. Before becoming a mom, she'd run eight marathons and was eight minutes shy of qualifying for Boston. "I had always been excited to get pregnant and had dreams of pushing my kid in a running stroller." But her regular runs came to a screeching halt 19 weeks into her pregnancy when she developed peripheral blood clots and had to reduce physical activity because of the pain. She'd been looking forward to lacing up her running shoes again, but after a difficult labor, she developed pelvic organ prolapse, a condition where weak muscles allow one or more pelvic organs (like the uterus or bladder) to drop down onto other organs or out of the vagina.