There can be more reasons why a gestational sac is not visible on ultrasound at 40 days of pregnancy. For example, menstruation is delayed by 7-10 days each time, and when it is delayed, ovulation is also delayed later, and thus implantation may be later. During ultrasound monitoring, the development of the gestational sac may not be visible. It is recommended to go back to the hospital for ultrasound at an interval of about 7 days, as long as the gestational sac, germ and fetal heart appear within 60 days, it is normal. Because everyone’s menstrual condition is different, the early or late detection of the gestational sac is related to the postponement of menstruation, including the early or late ovulation. If there is no delay in menstruation, the gestational sac can be seen normally at 40 days, and first of all, it is necessary to exclude the occurrence of ectopic pregnancy. If there is no mass in the vicinity, it is also necessary to review the pregnancy after a week interval. If there is no gestational sac in the uterine cavity at that time and there is a mass in the adnexa, it suggests the possibility of ectopic pregnancy.