18 weeks of pregnancy is about 4 months and 2 weeks of gestation. The month of pregnancy is not calculated by taking 30 or 31 days as 1 gestational month, but by taking 28 days as 1 gestational month and 1 gestational month as 4 weeks, so 18 weeks is about 4 months and 2 weeks of gestation. For women with regular menstrual cycles, the due date is usually calculated from the first day of the last menstrual period, and the month minus 3 or plus 9 and the number of days plus 7 is used to calculate the due date. However, for women with irregular menstrual cycles, instead of the above method, the due date is usually estimated by measuring the femoral diameter and head and hip diameter by ultrasound. After pregnancy, you should go to the obstetrics department of the hospital in time to establish a pregnancy book and have pregnancy tests on time and monthly, such as the Down screening test should be done at the hospital at 18 weeks of pregnancy. Therefore, it is recommended that pregnant mothers should pay attention to pregnancy checkups to avoid bringing danger to themselves and the fetus in the womb.