How many weeks pregnant is calculated from the first day of the last menstrual period, and the full length of pregnancy is about 280 days, or 40 weeks. Weeks of pregnancy are calculated from the first day of the last menstrual period, and every 7 days is a week of pregnancy. A full term pregnancy is approximately 280 days, or 40 weeks. Clinically, a pregnancy of less than 14 weeks is called an early pregnancy; 14 weeks and less than 28 weeks is called a second trimester pregnancy; 28 weeks and beyond is called a late pregnancy. If the last menstrual period is not clear, ultrasonography can be used to estimate it. If the last menstrual period is clear, ultrasound can also be used to calibrate the week of pregnancy. Generally, ultrasonography is used to measure the biparietal diameter, head circumference, abdominal circumference, and femur length of the fetus to make a comprehensive judgment of the week of pregnancy. If there is any discomfort, it is recommended that you seek medical advice and treatment.