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A prospective descriptive study was carried out to establish reference values for gestational age assessment in the first trimester by measuring embryonic length in dated human embryos, by using transvaginal ultrasound. In 160 pregnant women who had undergone assisted reproductive procedures, 139 singleton embryos and 46 embryos from 21 multiple pregnancies were studied between the 35th and 98th day after the last menstrual period. Transvaginal ultrasound examination, including inspection of embryonic structures and measurement of greatest embryonic length (L), was performed in these women. Estimation of gestational age (t) by measurement of L in singletons is best described by the equation t = 35.72 + 1.082L(1)/(2) + 1.472L - 0.09749L(3)/(2). Mean 95% confidence interval and 95% prediction interval were 4.3 and 9.3 days, respectively. There was no significant change in the estimation curve when multiple measurements in singletons were taken nor when embryos from multiple pregnancies were measured. Accuracy of embryonic age estimation from a single measurement of greatest embryonic length was evaluated.

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