It is possible for the number of days of pregnancy and actual conception to differ by three weeks. The number of days of pregnancy is usually counted from the first day of the last menstrual period. For women with normal menstrual cycles of 28-30 days, the calculated number of days of pregnancy and the actual number of days of conception are basically the same; however, for some women with long intervals between menstrual cycles, or those with disturbed menstrual cycles, the calculated number of days of pregnancy may differ from the actual number of days of conception, and a difference of three weeks may be possible. Using the last menstrual period to calculate the week of pregnancy is inaccurate in many pregnancies and needs to be confirmed or corrected by ultrasound. Usually the gestational sac can be seen on ultrasound around 5 weeks of gestation and the primordial heart tube pulsation can be seen on ultrasound around 6 weeks of gestation. However, it is most accurate to estimate gestational age by measuring the head and hip length of the fetus at 11 weeks-13+6 weeks of gestation. The week of pregnancy needs to be checked by a doctor.