The normal range of the indicators used for prenatal screening (alpha-fetoprotein and human chorionic gonadotropin) changes with each gestational week. So in order to determine if the results are in the normal range, a definite gestational week is necessary. If the gestational week is incorrect, then the risk may be very high or very low and not reflect the actual risk. If the menstrual cycle is accurate, the last menstrual period is remembered clearly, or even the timing of ovulation and sex is known for the purpose of planning a pregnancy, then the estimation of the gestational week should be easy. But if the menstrual cycle is inaccurate and the last menstrual period is poorly remembered, you will have to rely on an accurate ultrasound to make a proper assessment of the gestational week. If a doctor is presented with a high-risk report card, the first thing he or she does is also to recalculate the gestational week of the fetus to rule out a twin pregnancy. If the projected gestational week does not match the previous one, recalculate the risk again according to the exact gestational week.