An ultrasound in the first month of pregnancy may not always reveal the gestational sac. If the usual menstrual cycle is within 30 days, usually after 35 days of pregnancy, a small gestational sac can be seen inside the outline of the enlarged uterus by vaginal ultrasound. At this time, the size of the gestational sac is still relatively small, and there is no germ or fetal heartbeat inside, so it is not yet possible to fully identify an early intrauterine pregnancy and determine whether the gestational sac is viable. Generally, when the gestational sac is 6-8 weeks pregnant, the embryonic buds, amniotic level and fetal heartbeat can be seen inside the sac, then the pregnancy can be diagnosed as early intrauterine pregnancy and the sac is alive. If the usual menstrual cycle is more than 35 days, an ultrasound examination can only be performed after 50 days of pregnancy in order to observe the gestational sac. At 35 days of pregnancy, it is not always visible. Therefore, it is not always possible to detect the gestational sac on ultrasound when you are one month pregnant.