Quote from Phoebe in The One with the Embyros
Phoebe: Okay, I'm going to go to the doctor and see if I'm ready to have Frank and Alice's embryo transferred into my uterus. Ross: How will they know if you're ready? Phoebe: Oh, they'll just see if my endometrial layer is thick. Chandler: I can check that for you. Phoebe: Okay, everyone. Think "thick."