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Pregnancy development
It is likely that many eggs that are fertilised do not successfully implant in the womb, but are discarded along with other dead tissue cells. For this reason the legal definition of pregnancy in the UK is when a fertilised cell is implanted in the womb.
Many implanted eggs spontaneously abort, causing what is commonly known as miscarriage. This can take place at any point in pregnancy, but is most common in the first 12 weeks. Some miscarriages take place so early in pregnancy that the woman is not aware of her pregnancy yet.
The ‘primitive streak’ is a term to describe specific physical changes in the zygote that can be distinguished at about 14 days into pregnancy. After this time it is considered an embryo and UK law prohibits its use in experiments.
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