The primitive heart tube can be seen at more than six weeks of age. The fetal sac can be seen in the uterus at about five weeks from the first day of the last menstrual period. The criterion for seeing the fetal sac is the presence of a yolk sac, which is a confirmation of intrauterine pregnancy. One week later, at six weeks of gestation, the fetal heartbeat and fetal bud can be seen. The fetal heartbeat and fetal germ can be seen through ultrasound, but the time to hear the fetal heartbeat with Doppler is usually around 13 weeks of gestation, when the fetal heartbeat can be heard in the abdomen with Doppler instruments. The height of the uterine fundus at around 13 weeks of pregnancy is about 1-2 fingers above the pubic symphysis, and the fetal heart can be heard at this place by Doppler auscultation. The fetal heart rate at this time will be faster, and a fetal heart rate of 170-180 beats per minute is normal.