When sonographic evidence of an early pregnancy is definitive, the gestational age of the pregnancy can be estimated by measurements of the gestational sac, crown-rump length, or biparietal diameter. Mean sac size and crown-rump length provides the best estimates of gestational age during the first trimester. Measurements of sonographic structures very early in pregnancy give better estimates of gestational age than later measurements because the growth of the embryo during the first trimester is consistent between individuals and not dependent on genetic or nutritional factors. Mean sac diameter (MSD) is the average of three gestational sac measurements in millimeters: MSD = (length + width + depth) /3; gestational age (days) = 30 + MSD. In measuring crown-rump length (CRL) it is important to obtain the maximal embryo length, excluding the extremities and yolk sac. Gestational age (weeks) = 6.5 + CRL when CRL is measured in centimeters. Both MSD and CRL give very accurate estimates of gestational age. A significant discrepancy between CRL and MSD suggests a failing pregnancy. After the first trimester, gestational age should be estimated by measuring the (biparietal diameter) BPD of the fetal skull. The BPD is a transverse measurement of diameter at the level of the thalamus. The markers are positioned at the inside of one side of the skull and the outside of the opposite skull surface. Modern ultrasound software is capable of calculating gestational age automatically when the above measurements are marked on the display.
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