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Early pregnancy: coloured ultrasound scan showing the gestation sac (small purple oval, centre left, surrounded by red)) in the uterus at an early phase (4-6 weeks) of pregnancy. The solid purple region at upper right represents a full bladder; the pear-shaped body of the uterus is visible below it at bottom left. The cervix is towards the bottom right. At this stage of the pregnancy all the internal organs of the embryo such as liver, pancreas, stomach and heart have begun to form. It is during this period that the embryo is at the greatest risk of developing birth defects caused by abnormal genetic or external factors.
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