Douglas, I must disagree.
Excerpt from 'A child is born'*1 by Lennart Nilsson.
"At eight weeks, ..., the developing individual is no longer an embryo but a
foetus. Everything that will be found in the fully developed human being
has now been established.
The foetal stage is one of growth and perfection of detail. The heart has been
beating for a month."
Therefore I do not see how any such line could be drawn *after* this stage.
*1 Nilsson,L. 'A child is born' P71 0-571-111-394.(This is a really good read).