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The developmental age of an embryo in the first trimester of pregnancy is generally determined by ultrasound scanning and/or by calculation from menstrual age. In the original studies, validation of the estimate of gestational age by ultrasound was not possible as the exact date of conception was unknown. Variation in growth rates of identically aged fetuses has previously been reported after assisted conception and with the use of ultrasound scanning. As these pregnancies were ongoing the accuracy of the scanning results could not be determined. Comparison of scanning and direct measurements after termination of pregnancy and menstrual age were carried out to determine the accuracy in fetal dating. The results suggest that the use of ultrasound scanning to determine gestational age is of less use than previously thought, and that the use of menstrual age is severely limited.

Morphological and molecular characteristics of living human fetuses between Carnegie stages 7 and 23: ultrasound scanning and direct measurements