Daughter of a pharmacist meets a 1st world war fighter pilot, fall in love, have sex she falls pregnant, he is killed. She goes away to have baby comes back home then aranges for someone to leave baby boy on her doorstep thinking she can raise him but baby is given to another women. Mother leaves town goes to big city eventually becomes very wealthy making cosmetics.tries several times to get son back. Does get him on one occaision but boy misses his adopted father so mother allows him to go home. She stays in touch and helps him out a lot over the years. Ww2 comes and boy has enlisted but wants to marry before he goes. Because she is so wealthy and well connected she is able to arrange for some archbishop to forgo the fomalities and get him married before he goes to war. As he is filling in the forms he menyions he doesn't know his birthdate as he was adopted. She thens says "you were born on....." and he realises she is his real mother... A real tear jerker!

Possibly 'To Each His Own'? Here's a snippet of the plot:
She secretly gives birth to their son in 1919. She tries to arrange it so that she can "adopt" the boy without scandal by having him left on the doorstep of a family with too many children already, but the scheme backfires.

http://en.wikipedia.org/wiki/To_Each_His_Own_(film)

To "aliseaodo", many thanks, I have been trying to find the name of that movie for 35yrs,

:)