One kittens eye is still half sealed

My cat had 5 kittens Friday June 20 . They all seem to be doing extremely healthy and are all active. All of there eyes fully opened about 3-4 days ago. But one kittens eye is still half sealed shut. His other eye is fine, wide open. The eye that is half sealed shut doesn't look or seem to be infected at all. The eye ball underneath looks normal and I can see him looking at me with it. So I'm almost wondering if it's some sort of birth defect, can this happen? It's the outer corner of his left eye. He will be going to the vet next weekend, I'm just wondering if anyone has experience with this for piece of mind until we go.
I know kittens get eye infections but he doesn't seem like he has one, the eye is always clean, never goopy or pussy , or more squinty then before, that's why I'm so confused about it.

Any input?

If you have any eye ointment, you could put some in the eye to see if that will soften up whatever is sticking the eyelid shut.  Sometimes just wiping it with a cotton ball soaked in warm water will do the trick.  It could be something as simple as just a little dried discharge stuck in the hairs.  If that doesn't work, you might try putting something like vaseline over the part of the eyelid that is stuck.  It may soften it, or the mama cat may lick it off and open the eye in the process.  As long as there is no evidence of infection, don't worry.