If the lining is 0.9cm in 42 days after abortion, most people will have menstruation in a week or so, and the exact time varies from person to person. Normally speaking, the thickness of the lining is 0.8-1cm, which is the condition for menstruation, so if the thickness of the lining is 0.9cm and there is no abnormality, you can have menstruation in a week or so. If there is any abnormality such as residue in the uterine cavity or unevenness of the lining, then the time will be prolonged or irregular bleeding, and medication will be needed. There will be some damage to the lining after abortion. Therefore, in this case, the first period will be delayed, usually waiting for 2-3 normal menstrual periods to return to normal. If the period still does not come after a week, you need to check the ultrasound again, and according to the results of the examination, you need to follow the doctor’s advice to add medication to promote the onset of menstruation if necessary.