The normal range of the indicators used for prenatal screening (alpha-fetoprotein and human chorionic gonadotropin) varies with each gestational week. So in order to determine if the results are in the normal range, it is necessary to have a definite gestational week in order to do so. If the gestational week is not correct, then the risk may be high or low and not reflect the actual risk. If the menstrual cycle is accurate, the last menstrual period is remembered clearly, and even the timing of ovulation and sex is known for the purpose of planning a pregnancy, then the gestational week should be easy to estimate. However, if the menstrual cycle is inaccurate and the last menstrual period is not remembered, you will have to rely on an accurate ultrasound for a proper assessment of the gestational week. If the physician is given 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, the risk is recalculated according to the exact gestational week.