The earliest that a gestational sac can be formed is usually 20 days after intercourse, which means that it can be seen under ultrasound. The main purpose of gynecological ultrasound is to determine intrauterine pregnancy. The most important way to determine intrauterine pregnancy is to see the gestational sac in the uterine cavity, usually at 35 days after menopause, which is 20 days after sexual intercourse, is the fastest time when a round or oval gestational sac can be seen in the uterine cavity by ultrasound. If a woman has delayed menstruation, ovulation is a little later and if she does get pregnant, it may take about 30 days after intercourse to see the gestational sac. The fetal bud and primitive heart tube beat can usually be seen at six weeks of gestation, and at 14 weeks of menopause, the head and rump length of the fetus can be measured, allowing for a more accurate estimation of the week of pregnancy and correction of the due date. At 9-14 weeks of menopause, B-mode ultrasonography can rule out severe fetal anomalies such as anencephaly.