When does the mucus plug first form? During what week?

I am wondering when the doctor could see it during a pelvic exam; how soon after conception.When does the mucus plug first form? During what week?
As soon as you become pregnant the higher level of hormones starts making you have the cervical mucus which forms the cervical plug. So at around 3 weeks pregnant - 1 week gestation.
You Doc cannot see it; it's way too high for that, plus it's not exposed. The mucous plug sort of 'holds' the placenta, the baby, etc. inside your uterus. Think of a cork in a wine bottle.