Green’s Dictionary of Slang

to become pregnant.

1890-1904		Farmer & Henley Sl. & Its Analogues II 370/2: fall, verb [...] 2. (venery).—To conceive.
c.1900		‘Ramrod’ My Lustful Adventures (1966) 67: ‘[D]on’t imagine, Master Harry, this long spike has left anything behind it, unless you have planted a kiddy [...] If you have it doesn’t matter. She’s bound to fall sooner or later’ [Simes:DLSS].
1959		D. Hewett Bobbin Up (1961) 24: Thank Christ I’m too old to fall.
1962		C. Rohan Delinquents 150: Who would think I’d be so stiff as to fall again?
1967		N. Dunn Poor Cow 78: Do you know you can fall with a white man and have it with a coloured bloke when you’e carrying and it’ll turn out a half-caste?
1970		H.E. Bates A Little of What You Fancy (1985) 581: He didn’t think they fell nowadays, what with the Pill and all that.
1975		‘Buey’ Bush Contractors 246246: ‘I’m careful [...] but as you never know when you can fall, I always have them [i.e. ‘pills to bring on menstruation’] on hand’ [Simes:DLSS].
1981		J. Sullivan ‘Big Brother’ Only Fools and Horses [TV script] Do you know she was 39 when she fell for you?
1989		J. Morton Lowspeak.