i would wait abit longer cats can be late so unless she is in distress i would not take her to the vets, maybe it might be a good idea to get a cardboard box with some clean blankets and have her in your bedroom, sometimes its best to keep a cat in the one room they get less stressed looking for a "place" to give birth as they will hold out for that "safe place" as i said maybe your bedroom would be ideal,

just make sure when she does give birth that if you are able to be with her that you make sure

1- she brakes the sac's - if she does not you will need to do it and you will need to make sure the airways are clear
2- the after birth comes out after each kitten - she may eat it and thats fine let her
3- if she push's too much and for a long period of time and a kitten does not come out then consult your vet
4- make sure the room is warm, and quiet

above is just a few things you need to do or keep an eye on and i am sure plenty of people on here will add to it
good luck and let us know how she gets on and pictures too would be great !!