The NT test, also known as posterior cervical zona pellucida thickness measurement, is performed at 11-13 weeks of gestation to measure the thickest subcutaneous echogenic hyaline layer in the fetal nuchal collar by ultrasound to assess the possibility of structural abnormalities and chromosomal disorders in the fetus. The normal value of NT should be less than 2.5mm, if it is ≥2.5mm, you should be alerted to NT abnormalities, and further diagnostic tests such as non-invasive DNA testing and amniocentesis can be performed as prescribed by your doctor. If it is greater than 6mm, the fetus is at higher risk of Down’s syndrome or structural heart malformation, and the pregnancy should be terminated if necessary if the fetal developmental malformation is confirmed. In addition, the week of pregnancy is calculated from the first day of the last menstrual period. 7 days of pregnancy is one week of pregnancy, 28 days is one month of pregnancy, and the total number of days of pregnancy is 280, i.e. 40 weeks. However, if a pregnant woman usually has irregular periods, irregular cycles, or unexpected ovulation, the gestational week needs to be determined in conjunction with the results of ultrasound examinations. Generally, during the early 3 months of pregnancy, you can combine ultrasound to measure the size of the fetal bud, the time of menopause, menstrual cycle, and the time of intercourse, so that it is more accurate to check the expected delivery date.