It's a staple of sub-par sitcoms involving pregnant women. The mom-to-be begs her significant other to make a midnight run to the 7-11 for pickles and ice cream or some other weird food combination.

Real life doesn't come with a laugh track, but pregnancy does often involve food cravings. These cravings are not limited to foods like pickles and ice cream, of course; pregnant women have been known to crave all kinds of foods, from blueberries to hamburger to cookie dough. Since such cravings have not been studied extensively, science has yet to deliver a fully convincing explanation. But there are several likely hypotheses.

According to one psychological theory, pregnant women crave specific foods because they're been taught to believe that that's how pregnant women behave. If you're supposed to crave pickles, then you will crave pickles.

However, most scientists recognize that pregnant women develop heightened senses. This means that while some foods may smell and taste better than normal, some typically attractive foods may become repulsive. There's even a condition called "pica" that makes normally non-edible substances such as dirt, chalk, clay, and even toilet paper, suddenly irresistible.

Should pregnant women give into urges? According to nutritionists, to maintain a healthy diet pregnant women should eat about three hundred more calories than usual. It's even ok to eat more than three hundred extra calories, as long as those calories are spread evenly across the important food groups. So go ahead and indulge food cravings in moderation, but don't eat five gallons of ice cream just because it tastes especially good. And in any case, it's best to lay off the toilet paper.