Answer:
   
   Forty days after a child is conceived, the Talmud tells us, the soul
   enters the body. Forty, therefore, is a symbol of birth, rebirth and
   change. It is interesting to note that it was recently found that
   neural activity does begin at 40 days after conception. This also
   means that abortion is permitted in more instances within the first 40
   days of pregnancy than during the rest of pregnancy.
   
   For the same reason, ritual immersion is done in a minimum of 40 seah
   (a unit of volume) of water. Note that the letter mem, whose name is
   from mayim (meaning water or fluid in general), is 40 in gematria.
   When God wanted to rebirth the world, it rained for 40 days causing a
   flood. Similarly, the Jewish People were born during 40 years in the
   desert.