I used oratane from june 2018 - i stopped taking the tablet mid to end august 2018. i found out i am 5-6 weeks pregnant which means i fell pregnant between 5-12 sep 2018. will there be any risk of my baby having birth defects?

Unlikely ...but..: The half life (time for half to be gone) of this drug is 20 hours and it is considered effectively gone from your body after 4 half lives (80 hrs). Since you cite possible conception more than 120 hrs after stopping it is likely not a factor. You may want to consult with a geneticist , usually found affiliated with Med schools or children's hospitals for further guidance.