It's all about context
What time of day and where you meet his mom and dad set the tone for how you should dress. For example, if you're meeting for dinner at a nice restaurant, showing up in baggy jeans and an old tee sends the message that you haven't made an effort and don't really care. Conversely, if you're meeting for a casual breakfast and you show up in a sequin mini, you may not convey the taste level they are looking for in a future daughter-in-law.
Be yourself (your BEST self)
It's easy to get so caught up with trying to dress to impress that you forget that what your man really wants is to introduce his parents to YOU! There's no sense dressing like a Stepford Wife when they're bound to discover that you're actually a jeans and a T-shirt kind of girl. Not only is authenticity key, but it sends a message to your man that you're confident with who you are.
Instead of undergoing a complete fashion personality transplant, work with your existing style and just refine it a little. A good way of doing this is to find a celebrity that has a similar style to you. See how they dress up or down according to where they are. Then find your own way of doing the same so that his parents meet the real you (only the best version of her).
Be a lady
You might have cleavage that drives their son wild or thighs that would make a young Jane Fonda envious, but you don't have to share these assets with his parents. Whatever you end up wearing, make sure that you never show too much skin or you'll risk misrepresenting yourself and alienating everyone (especially his mother!). Stick to the simple rule of dressing like a lady by asking yourself, "Would they allow this outfit on breakfast television?"
This means no super short skirts, nothing see-through, no breast-baring necklines and nothing two sizes too small. Remember, you want them to be focused on your natural charm and personality, not your cup size. Instead opt for a universally inoffensive look like this flattering dress by J. Crew ($170). And make sure your outfit is clean. Showing up with old stains and cat hair sends the message that you're a slob and won't make for a good wife or mother.
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