Moshe Rabeinu came down from Har Sinai after 40 days.
We came out of the midbar after 40 years.
We come out of a mikvah of 40 se’ah.
The gender of a fetus is set after 40 days.
I would philosophically suggest that 40 days represents the time it takes to become something new. ie.
After a 40 days ‘pregnancy’ we became the Jewish people.
After a 40 year ‘preganacy’ we reached a level as a whole to be zocheh to enter E”Y.
The entire concept of mikvah is based on pregnancy and rebirth.
A fetus becomes a he or a she after 40 days.
I’m sure it is much deeper but I’d imagine its something along these lines.