Pregnancy dates are usually counted from the first day of the last menstrual period for those with regular periods. Those who have irregular periods will need an ultrasound to assess the week of pregnancy. Gestational period is the period of time between conception and delivery, also known as the period of pregnancy. For ease of calculation, the date of gestation is usually counted from the first day of the last menstrual period, with full-term pregnancy being 37 weeks onwards. The entire course of pregnancy can be divided into three periods: early pregnancy, which refers to the beginning to the end of the 12th week of pregnancy; mid-term pregnancy, which refers to the end of the 13th to the end of the 28th week; and late pregnancy, which refers to the end of the 29th to the end of the 40th week. When the date of the last menstrual period cannot be specified, the week of pregnancy can also be assessed by ultrasound measurements of the fetus (e.g., fetal head-buttock diameter, biparietal diameter, head circumference, femur length, etc.).