The "count the wet diapers" advice worked great in the era of cloth diapers, but it can hard to tell if an ultra-absorbent disposable diaper is moist -- especially because a newborn may pee as little as one tablespoon at a time. Still, it's important to keep track, because if your baby has fewer than six wet diapers in a 24-hour period, it could mean she's not getting enough breast milk or formula, putting her at risk for dehydration and inadequate weight gain.
One way to know if your baby has urinated is to place a square of toilet tissue in each fresh diaper and then check to see if it's wet. You can also do the pinch test: If the diaper feels the slightest bit mushy, it's holding liquid. You can also try using a lower-priced disposable until you're sure your baby's gaining weight -- the cheap ones are less absorbent, so it's more obvious when the diaper is wet. Of course, you can also use cloth diapers for a few weeks (you'll have no trouble knowing whether or not one of those is wet).
However, you shouldn't rely merely on wet diapers to gauge your baby's health. Look at her: If she seems listless; has a weak cry; has a sunken soft spot or dry lips; or her skin doesn't spring back when you pinch it, she may be dehydrated and require emergency medical attention. Finally, pay attention to your gut feelings: If you're worried for any reason that your newborn is underfed, take her to the pediatrician immediately --Katy Koontz
Originally published in Parents magazine, August 1999. Updated 2009.