My wife is 5 month pregnant, is it safe to have sex with her? if yes then what are the positions and up to which month we can have sex?

Ok with caution: It is safe to have sex provided that it doesn't cause discomfort on her. Cervix becomes softer during pregnancy and is more fragile that non-pregnant women, so be extra cautious.

Being Pregnant: There are no restrictions on sex or positions during pregnancy except in the rare specific high risk pregnancy that your wife's OB would have verbalized specifically.

Sex: The good news, depending on how you look at it, is that sex during pregnancy is extremely safe for most women with uncomplicated, low-risk pregnancies, and can occur up to the last few weeks. Certainly the last few weeks can be more difficult and uncomfortable. Use common sense and avoid rough sex in the third trimester. Positions are whatever is most comfortable for the pregnant woman.