Foetus

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The term Foetus (or fetus) is defined by the Cambridge dictionary as being "a young human being or animal before birth, after the organs have started to develop" [1] It is distinct from an embryo as the term embyro more commonly refers to a pre-birth organism that has yet to begin developing its organs; that is, embryos are at an earlier stage in development to foetuses. In humans, unborn offspring are thought of as embryos until the eighth week of gestation, after which it is termed a foetus [2]. It is during the embryonic stage that cells divide and begin to differentiate into tissues, and in the foetal stage that the rest of development takes place. The offspring is a fetus from when the embryonic stage ends right up until birth occurs. Despite this, the terms are often used almost interchangeably outside of the scientific world.  In humans, the development of the fetus takes place over the last three trimesters of pregnancy. At the initial starting point of the foetal stage (approximately nine weeks into pregnancy) the foetus is on average around 30mm in length. Limb extremities (i.e hands and feet) are present, as are some organs, but these have a minimum level of function at this early stage. In the next trimester (at seventeen weeks) foetal movements gradually become noticeable by the parent. By this stage the foetus is generally 18-20cm in length. The final trimester (26 weeks) is the most crucial, as it is at this point that most of the organs develop enough to become functional, bones develop, and there is an increase in crucial minerals such as calcium, phosphorous and iron. The foetus is 48-53cm in length in this stage. This trimester ends when birth occurs [3].  In other animals the development follows a highly similar pattern, but the length of time that the process takes varies, as do the tissues and morphology of the organism. 

References

  1. http://dictionary.cambridge.org/dictionary/british/foetus
  2. http://pregnant.thebump.com/pregnancy-week-by-week/4-weeks-pregnant/qa/difference-between-embryo-and-fetus.aspx
  3. http://en.wikipedia.org/wiki/Fetus#Weeks_9_to_16
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