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Major improvement in the ultrasound assessment of early pregnancy came up with the introduction of transvaginal ultrasound at the end of 1980s. High-frequency transvaginal transducers improve the image quality to an extent that detailed description of the embryonic morphology became possible with in-depth anatomical studies of the brain compartments, the spine, the heart, the stomach, the midgut herniation and the limbs. There are three main characteristics that mark the early human conceptus: its small size, its rapidly changing anatomical appearance and its uniform development and constant growth. Embryologists use the Carnegie staging system to divide the human embryonic period into 23 developmental stages, commencing with fertilisation at stage 1, continuing into the fetal period with the onset of marrow formation in the humerus after stage 23, which takes place at 56-57 days post-ovulation, and the designation 'embryo' is replaced by 'fetus'.
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