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Human embryonic development covers the first eight weeks of development, which have 23 stages, called Carnegie stages. At the beginning of the ninth week, the embryo is termed a fetus (spelled "foetus" in British English). In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs.
A fetus or foetus (/ ˈ f iː t ə s /; pl.: fetuses, foetuses, rarely feti or foeti) is the unborn mammalian offspring that develops from an embryo. [1] Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus.
Images of embryos provide a means of comparing embryos of different ages, and species. To this day, embryo drawings are made in undergraduate developmental biology lessons. Comparing different embryonic stages of different animals is a tool that can be used to infer relationships between species, and thus biological evolution .
Obstetric ultrasonography, or prenatal ultrasound, is the use of medical ultrasonography in pregnancy, in which sound waves are used to create real-time visual images of the developing embryo or fetus in the uterus (womb).
The book was often cited as presenting the first images of a live fetus in utero. [14] In fact, Geraldine Flanagan's The First Nine Months Of Life had in 1962 compiled a similar set of fetal images from medical archives. [15] The images played an important role in debates about abortion and the beginning of human life. [16]
A newly developing human is typically referred to as an embryo until the ninth week after conception, when it is then referred to as a fetus. In other multicellular organisms, the word "embryo" can be used more broadly to any early developmental or life cycle stage prior to birth or hatching .
Prenatal development (from Latin natalis 'relating to birth') involves the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization , in the germinal stage of embryonic development, and continues in fetal development until birth .
Fertilization is the event most commonly used to mark the beginning point of life, in descriptions of prenatal development of the embryo or fetus. [28] The resultant age is known as fertilization age, fertilizational age, conceptional age, embryonic age, fetal age or (intrauterine) developmental (IUD) [29] age.