In the first week an entire human being is still a ball of cells under 0.2 mm across. Forty weeks later that figure passes 37 cm, a span of growth only a logarithmic scale can hold: the zygote cleaves into a blastocyst, implants in the uterine lining, forms three germ layers, and in weeks 3 to 8 the neural tube closes while the heart starts to beat. That same window is the most vulnerable one, when every major organ is taking shape.
In the first week an entire human being is still a ball of cells under 0.2 mm across. Forty weeks later that figure passes 37 cm, a span of growth only a logarithmic scale can hold: the zygote cleaves into a blastocyst, implants in the uterine lining, forms three germ layers, and in weeks 3 to 8 the neural tube closes while the heart starts to beat. That same window is the most vulnerable one, when every major organ is taking shape.
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