It has been around for quite a long time - from OED
b. To become pregnant.
1722 Session Bk. Penninghame (1933) I. 479 The said_Jannet_confessed that she fell with child and parted with it in May last.
1891 Farmer Slang II. 370/2 Fall (venery), to conceive.
1957 Young & Willmott in _C. H. Rolph' Human Sum vii. 129 The expression a woman uses when she is pregnant. She says she has _fallen'. _We had been married eight months before I fell.'
This is one of the peculiarities of English that make it so hard for foreigners to understang what we are talking about.
I remember a story of a woman going to see a non British doctor and saying ''I think I have fallen again Doctor'' the Doctor though he should be checking for fractured bones.