Once the egg has been fertilised the development of the offspring begins. It must also implant into the lining of the uterus to ensure protection and a continued supply of nutrients.
An animation of foetal development and associated risks is shown below:

Foetal development

A good link to view the developmental changes week by week is shown below.

Developmental stages...

and here:
Week by week...

Review the stages in the Hirschfeld archive:
Pregnancy

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