Pregnancy and treatments

I am new to this site and looking for some information and advise. I have had Psoriasis since my teens and managed it well with topical treatments, however since having my son 18 months ago it has become progressively worse and covered 75% of my body. I have been undergoing light treatment and have exceeded the normal 30 treatments. The treatment has worked wonders however it has not completely cleared and during a recent two week holiday has come back in some areas. I have been advised that I will probably have to take oral medication for it (waiting for appointment to discuss this with my consultant) but concerned about this in relation to having another child. The research I have done so far suggests that I cannot take this medication if pregnant or intend to become pregnant. Has any one been in this situation or have any advise?