My wife is running 9th month is safe now to have sex

What problems can come if I have sex at that time

Sex is safe during any time of pregnancy.

So long as your wife doesn't have a high risk pregnancy, and her doctor says that it is fine, there is no reason that sex shouldn't be safe during pregnancy. In fact, it can be even more fullfilling during pregnancy, emotionally and physically. If you have concerns, discussing them with your wife's healthcare provider to ease your fears

Heck, some docs recommend it right up until the end. They say that the sperm can soften the cervix and begin labor. Is this an old wives tale? I am not sure, but yes, sex is safe during all phases of pregnancy unless, as Buggage said, she is not high risk.