The calculation of the pregnancy cycle for women with regular menstrual cycles is based on the last menstrual period, which is extrapolated to one week in seven days. If you have irregular menstrual cycles, you can use ultrasound to calculate your menstrual cycle, which is more accurate than the last menstrual period, because it can exclude the influence of the late birth on the week of pregnancy. The entire gestation period is 40 weeks, and a full-term fetus is born after 37 weeks of gestation, while a fetus born at less than 37 weeks is considered premature. You can also use the hormone level in the blood to calculate the pregnancy week, the accuracy is relatively high, because of the complexity of the test can not be timely results are generally not used, only as an approximate reference. The first two periods of pregnancy need special attention, especially the early pregnancy, that is, before the 12th week of pregnancy, because the fetus is unstable during this period is prone to miscarriage, and the late pregnancy needs special attention prone to preterm delivery, these two periods pay attention to more rest and avoid strenuous exercise.