Anal distension at 21 weeks of pregnancy may be caused by physiological factors, constipation, pelvic inflammatory disease and other factors. 1. Physiological factors: after pregnancy, due to the gradual increase in the size of the uterus, may cause pressure on the rectum, which will produce anal swelling, especially after standing for a long time, anal swelling is more obvious. 2. Constipation: during pregnancy, affected by the hormone level changes in the body, the pregnant woman’s gastrointestinal digestion and absorption capacity deteriorates, and the increasing uterus to the intestinal tract pressure, often constipation, constipation is serious, it will produce anal swelling, accompanied by defecation difficulties. 3. Pelvic inflammatory disease (PID): During pregnancy, women’s immunity is low, improper care may induce pelvic inflammatory disease (PID), which may cause anal distension and swelling, and may be accompanied by lower abdominal pain and irregular vaginal bleeding. During pregnancy, pay attention to rest more, at the same time should relax, avoid tension, anxiety, any discomfort should be timely to the hospital, by the doctor to clarify the diagnosis, and cooperate with the doctor to deal with the symptoms.