The porn movie that Joey and Chandler were watching when the super comes in is "On Trial: Oral Arguments."
When Phoebe is at her ob/gyn the doctor has a name tag for Dr. Oberman. That's the same name as was Carol's ob/gyn in The One with the Sonogram at the End (1994). Also The One Hundredth (1998) has a male ob/gyn with the same name.
The Magna Doodle reads "Knock Knock Who's there? PORN!"
In the extended version there is an alternate ending where Phoebe, Frank Jr. and Alice are all in the OB/GYN office with Phoebe getting another ultrasound, and Frank exclaims that he likes the middle baby in the ultrasound the best, and Phoebe and Alice look at him pointedly to which he says that he won't let the other two babies find out about his preference.