How Can Ultrasound Determine Gestational Age?
Usually, determining your baby’s gestational age and your due date is calculated from the first day of your last menstrual period (LMP). If your last menstrual period is unknown, an ultrasound evaluation of fetal size can determine gestational age and due date. Ultrasound determination of gestational age is most accurate during the first trimester because there is little biological variability in size from one pregnancy to the next and few factors that significantly affect fetal growth. Measurement of the fetal pole from one end to the other is called the crown-rump length (CRL). This measurement correlates with actual gestational age within five to seven days. Typically, if the due date suggested by the CRL falls within about five days of menstrual dating, the due date established by the LMP is kept throughout pregnancy. If the due date suggested by the CRL falls outside this range, the due date from the ultrasound is usually kept. Ultrasound is particularly helpful in establishing ge
