For first time pregnant women or normal pregnant mothers, many people do not know when to do the ultrasound, and some people, for a long time without menstruation also do not feel any problem, until the belly is big to come to the hospital, only to find out that the pregnancy, that ultrasound can find out how long is the pregnancy? The size of the gestational sac directly affects the development of the fetal embryo, and there are some differences in the projection of the gestational week by ultrasound in different stages of pregnancy. The first day of the last menstrual period is usually used as the first day to start counting the weeks of pregnancy. If the size of the gestational sac does not match the last menstrual period to project the gestational week, it is recommended that the development of the fetal heart and germ should be observed, dynamically, and attention should be paid to regular visits to the hospital to check the development of the fetus. At the earliest, the gestational sac can be seen on the 35th day of pregnancy, but the time of conception also varies depending on individual differences, and some pregnant women may delay it until about 45 days. The development of fetal heart and germ can be seen around 46 days after menopause, so it is recommended to perform ultrasound after 46 days to check whether there is a fetal heart or not. For pregnant women with irregular menstruation, they can expect 7-10 days before performing ultrasound. Before 12 weeks, the method of estimating the fetal bud is actually the same as the head-rump diameter, and this is the most accurate period for estimating the gestational week. If a pregnant mother does not have an ultrasound examination before 12 weeks, she can roughly estimate the gestational week based on the size of the biparietal diameter, but because the fetal growth in late pregnancy is affected by various aspects, the ultrasound projection of the gestational week at this time is not accurate. For pregnant mothers with normal pregnancy and normal maternity checkups, the fetal growth is continuous and the difference in the value once does not reflect the fetal development during the whole pregnancy, so as long as the measured value is within the normal range, pregnant mothers can relax, adjust their diet, exercise moderately and review the ultrasound regularly.