Jeans styles come and go -- but some styles are best to avoid altogether. There are certain jeans fits that are so universally unflattering, they’re known by pejorative nicknames. Here's a guide to Mom Jeans, Dad Jeans and Grandpa Jeans, so you can know what not to wear when it comes to denim.

Mom Jeans

We can thank Tina Fey for making most of us aware of the sartorial dangers of "mom jeans". These unflattering denim bottoms were worn by women for years before the comedienne made them famous in a 2003 Saturday Night Live skit.

It advertised a fake brand of jeans called Mom Jeans, using the tagline, "For this Mother's Day, don't give Mom that bottle of perfume. Give her something that says, 'I'm not a woman anymore...I'm a mom!'"

So what do mom jeans look like? They typically feature a solid, light blue wash with no fading or distressing. They are high-waisted, with a rise that hits above the belly button, making a woman's bottom appear longer and flatter than it otherwise might. Worst of all is the extra fabric around the zipper, sometimes with pleating, which magnifies any hint of a poochy tummy. Tapered legs and elastic waistbands are among the other (sometime) horrors of the mom jeans style.

Before the mid 1990s, when lower rise jeans became more popular in the United States, plenty of women wore their mom jeans and tucked-in shirts with apparently no idea of how bad they looked. These days though, there are hundreds of jeans styles on the market -- including plenty of medium rise, dark wash styles to suit stylish, mature women.

So there’s no longer any excuse for being caught wearing mom jeans.

Dad Jeans / Grandpa Jeans

Men's jeans trends don't change as much with the fashion seasons as do women's styles. Nevertheless, there are certain types of jeans no self-respecting man should ever be caught wearing. A case in point are so-called "dad jeans" -- those unfashionable denim trousers that feature a light blue wash, higher waist, fuller legs and baggy behind. Named for the suburban dads you can picture wearing them, these jeans are unflattering to pretty much everyone and are old school in the extreme. These jeans got a lot of media attention in 2009, when President Barack Obama was photographed wearing the shapeless style to a baseball game. Some people call this style "grandpa jeans", just to rub in their unfashionable status.

In the few years since, Dad jeans have been co-opted by a few, uber-cool pockets of urban hipsters who dig the irony of wearing jeans so uncool they're cool. Some even add thin, rolled cuffs to these jeans. But if you don't live in Portland, Oregon or Toronto's Queen West Village, you should probably skip this denim style. The fact is, these hipster "dad jeans" are usually stylized versions of the real thing, that have been cut to a fit a younger, slimmer frame.

Meanwhile, there are plenty of American men out there who never stopped wearing faded-out dad jeans. They're the guys who buy their jeans at discount stores without even trying them on -- and it shows. Do yourself a favor if you're a guy who cares about looking current and stick to dark wash jeans that fit your rear end. While guys can get away with a slouchy pair of jeans, to look modern you need to have that fit in the seat and a slim to straight cut style through the legs.