Menstruation 20 days after cesarean section is usually abnormal and may be due to bleeding caused by poor uterine contractions and residue in the uterine cavity. The time of menstruation after cesarean section is not fixed, affected by breastfeeding, menstruation may be delayed, some women may need 2-3 months to have menstruation, but some women even never have menstruation for the whole breastfeeding period. If menstruation occurs more than 20 days after cesarean section, it is recommended to go to the hospital for ultrasound examination of the uterus in a timely manner to determine the recovery of the uterus as well as whether there is any residue in the uterine cavity. If it is found that the uterus is not recovering well, or there are residuals in the uterine cavity, it is recommended to use drugs to promote uterine contraction and facilitate the discharge of residuals. If there is no abnormality in the test results, the onset of menstruation does not require special treatment and observation. If menstruation occurs 20 days after cesarean section, it is recommended to consult a doctor in time and determine whether it is normal according to the results of relevant examinations.