How to calculate pregnancy days accurately

The number of days pregnant is calculated from the first day of the last menstrual period, 2 weeks before ovulation or fertilization, and 3 weeks before the baby's implantation. For abnormal menstrual pregnant women can be ultrasound examination of gestational sac to calculate gestational weeks, for regular menstrual cycle pregnant women, the whole process of pregnancy is 40 weeks, that is, 280 days, for irregular menstrual cycle pregnant women can not use the first day of the last menstrual period to calculate, but according to early pregnancy B ultrasound to calculate. During pregnancy to do NT value screening time B ultrasound is relatively more accurate, according to the length of the fetal crown hip diameter plus 6.5 is the gestational week, the specific number of days of pregnancy by the doctor according to the pregnant mother's menstrual cycle and early pregnancy B ultrasound, calculated is the most accurate.