If you are pregnant after 1 month of intercourse, the gestational sac will be about 1.5-2.5 cm, but this value is for reference only, because the size of the gestational sac is determined by the early and late embryonic implantation and the development of the embryo, and also varies depending on the pregnant woman’s physical condition. Clinically, the gestational week is calculated from the first day of the last menstrual period, 30 days after intercourse, plus the previous 15 days between the last menstrual period and the time of ovulation, which is about 45 days after the last menstrual period. If the pregnant woman has a more regular menstrual cycle of 28-30 days and the embryo is well enough developed to go for an ultrasound examination at 45 days of pregnancy, the size of the gestational sac is likely to be more than 2 cm. If the pregnant woman’s menstrual cycle is 30 days or more and irregular, and the embryo development is not good enough, the gestational sac is likely to be less than 1.5 cm, and some pregnant women may not even see the gestational sac. At this time, it is recommended to do a blood HCG test first to initially determine the specific reasons for not seeing the gestational sac (late embryo implantation, pregnant women not holding enough urine, ectopic pregnancy, etc.). The test can also be repeated after 2 weeks to check the size of the gestational sac and to correct the gestational week at the same time.