If the pregnancy measured 12 weeks, the true gestational age was 12 ± 0.96 weeks. Therefore, if a pregnancy measured 8 weeks 0 days, the true gestational age was 8 weeks 0 days ± 0.64 weeks. 11 showed that the variability in the CRL remained fairly constant from 2 mm to 12 cm, where the 95% confidence interval for gestational age was 8%. From 11 to 13 weeks, the 95% confidence interval is within 5 days. 9 When examining pregnancies conceived through IVF, where the precise gestational age is known, from 7 to 11 weeks, the 95% confidence interval shows that the CRL is within 3 days of true gestational age. As the pregnancy advances, the CRL loses precision in gestational age determination. The accuracy of gestational age determination based on CRL is best in the middle of the first trimester. B: The CRL at later gestational ages (GA) allows for a clearer distinction of the fetal edges. Calipers are placed from the tip of the head to the edge of the caudal end.
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A: The CRL is measured in a sagittal plane through the fetus. When obtaining the CRL, the sonologist should obtain at least three measurements and average the values to obtain the gestational age (see Table 1 in Appendix A1).įIGURE 3.3: Crown rump length (CRL). Figure 3.3 depicts measurements of the CRL at different gestational ages. In truth, the CRL is the measurement of the longest straight line from the head of the fetus to the caudal end. The CRL is not the true measurement of the length of the fetus, as fetuses in early gestation have flexion. Once an embryo is seen, the measurement of the crown rump length (CRL) can be obtained, and this is now more accurate in obtaining the true estimation of gestational age. Between 2 and 14 mm, the MSD is quite precise in determination of age, but after a MSD of 14 mm, an embryo can be seen. The sac also has a hypoechoic rim around a hyperechoic sac wall.Īs the first trimester advances, the reliability of the MSD in predicting menstrual age decreases. It is displaced inferiorly such that it is located in the decidua rather than the endometrial canal (line). In this image, the gestational sac is seen eccentrically located in the endometrial cavity. 3.2).įIGURE 3.2: Characteristics of a gestational sac.
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Also, the intradecidual sign can be seen, where the sac is imbedded in the decidua rather than between two layers of the endometrium ( Fig. The deep layer of the decidua parietalis and early villi is more echogenic, and this can be seen as separated from the less echogenic layer of the superficial layer. This is a finding that is created by visualization of the two layers of the decidua parietalis. To distinguish a true gestational sac, one must identify a double sac sign. The gestational sac can often be confused with a pseudo-sac seen in ectopic pregnancy. The MSD is the average of the three measurements. Notice that the sac wall is not incorporated in the measurement. The gestational sac is measured in three planes with the calipers placed at the interface between the wall of the sac and the embryonic fluid.