A 1.0 x 1.6cm gestational sac is about 28~35 days after intercourse. A 1.0×1.6cm gestational sac is around 6~7 weeks of pregnancy. The last menstrual period is calculated to be around 42~49 days of pregnancy, and the day of ovulation is around 14 days before the next menstrual period, and conception by intercourse on the day of ovulation should be presumed to be around 28~35 days after intercourse. Multiple intercourse during ovulation and the development of the gestational sac can cause inaccurate pregnancy time projections. The size of the gestational sac is related to the number of days of pregnancy, the hormone level of the pregnant woman’s body, the development of the embryo and many other factors, according to the size of the gestational sac to estimate the gestation period may appear larger error. Early pregnancy ultrasound is the most accurate way to estimate the week of pregnancy. It is recommended to measure the length of the embryo by ultrasound and estimate the week of pregnancy according to the length of the embryo.