Zhukov, now a lieutenant-general, is assigned to the First Soviet Mongolian Army Group in July. After being given command of the group he prepares to take on the Japanese. The conflict climaxes at the Battle of Khalkin-Gol, beginning on 20 August.
If you have a normal 28 day cycle, you are most likely to become pregnant when you have unprotected intercourse approximately 2 weeks before you anticipate your period to start.
If however, you've had a history of regular menstrual periods, you should get a test if you miss your period. Or do you mean how long after your period should you be worried about getting pregnant through unprotected sex? You are referring to the calendar method.