STEVE ALLEN / SCIENCE PHOTO LIBRARY STEVE ALLEN / SCIENCE PHOTO LIBRARY
Foetus at 12 weeks. At this stage of development (end of the first trimester) the foetus measures about 6 centimetres in length and weighs about 20 grams. At 12 weeks the placenta is fully- developed, and the foetus is able to move its arms and legs. The eyes have formed, but the eyelids remain sealed.
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