Hopefully your birth will go well - PFFD will have no effect on the delivery.
You will probably at birth be asked if you want to have the placenta taken, or the parents blood taken, or the baby's blood taken for "genetic testing." This is apparently a question parents of PFFD babies get asked a lot by pediatricians and OB doctors who are unfamiliar with PFFD. Note: PFFD is not a genetic effect but an exposure effect. (see Etiology - what causes it?
If PFFD wasn't detected by ultrasound and you haven't already talked to an orthopaedic specialist you may be offered the opportunity to talk to one now. Relax. PFFD is not an emergency - there isn't anything that a surgeon wants to do at this point. The earliest you will have to do something is when baby starts trying to walk and even then at that point it's probably just prosthethics. You have some recovery from the birth to do and enjoyment with the baby. In fact a newborn baby's legs are so short anyway that if you have unilateral PFFD it will hardly look like one leg is shorter than the other even with a 20% discrepancy.