Embryonic ultrasonographic measurements: crown-rump length revisited.

New endovaginal probes will result in many investigators reexamining the crown-rump length data first generated with static arm and abdominal real-time scanners. As new tables are produced, only through better understanding of the anatomy of early embryos can we realize what we are actually seeing and measuring (or at least attempting to measure). Furthermore, as we try to correlate early embryonic size with gestational age, we realize that the crown-rump measurement is not applicable in principle or in nomenclature when the embryo is less than 18 mm long. When first seen, the embryo is relatively straight; a measurement of its size is best described as "greatest length." As the embryo curves into a C-shaped, tadpolelike structure, the greatest measurement along the long axis is actually a neck-rump measurement. Further unfolding of the head and regression of the tail finally allow an accurate crown-rump measurement (at about 18 mm) as we have used it for almost two decades. Early embryonic stages and characteristics are reviewed and methods of measurement of early embryonic size are explained.