The first trimester is the most critical time for an embryo or fetus.
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It is true that the first trimester is the most critical time for an embryo or a fetus. This is because there are many crucial developmental stages in this time period in order to produce a viable and functional multicellular organism. Such developmental characteristics in this period include the successful formation of the notochord, the basic nerve structures and the beating heart. It is critical for such structures and organs to form properly in order to ensure subsequent developments in the fetus to form correctly and ultimately result in a successful deliverance.