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Week 4

By four weeks, the embryo has a head, tail, backbone and limb buds - which will eventually become arms and legs. The beginnings of ears and eyes are also visible. Its heart is already beating, and the other organs are forming fast. An umbilical cord starts to grow between the embryo and the placenta. During this time, the embryo is especially sensitive to any drugs or infections capable of crossing the placenta from the mother.

A four-week-old human embryo.
A four-week-old human embryo.
MRC Human Embryo Bank / Wellcome Photo Library

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