Pregnancy Prepositions

In the United States, one says, she is pregnant BY him...

In Ghana, one says, she is pregnant FOR him...

This may be simply a random twist in syntax, but to me the different prepositions carry different connotations.  To be pregnant BY someone seems more to imply that the man is the subject and the woman the object, while to be pregnant FOR someone seems to imply more agency on the woman's part; she is the subject and he the object.  In addition, to be pregnant FOR someone suggests that a woman is doing a man a favor by carrying his child. 

I'm warming up to the use of "FOR" in this case.