The absence of menstruation accompanied by low back pain is not necessarily a sign of pregnancy, because under normal circumstances, low back pain may be caused by congestion of the uterine cavity or by the posterior position of the uterus before menstruation. Therefore, you can’t judge whether you are pregnant simply by the absence of menstruation and back pain. If you want to judge whether you are pregnant or not, generally speaking, you should test with pregnancy test paper or pregnancy test stick about 16 days after intercourse, or go to the gynecology department of the hospital about 11 days after intercourse to check the blood HCG to confirm whether you are pregnant or not, instead of judging whether you are pregnant or not by the manifestations of waist pain or stomach pain.