I think the phrase may have been coined from the days prior to scans/home HPT's etc etc....

My Grandmother on my Dad's side had "phantom pregnancies"....my Aunt has her diaries where she talks about it: very very sad. (She had had 2 children, then her 3rd and 4th pregnancies were considered phantom, and by reading through, she was very sick in between & very depressed....)

In hindsight the family think that it was most likely blighted ovum/missed mc's.....but she lived in a rural village in the UK through her childbearing years in the 1930's.....so I think she most likely WAS pregnant, but then the pregnancies failed to grow, and they were labelled by her GP as a "phantom pregnancy". Whereas these days, at least we have access to HTP's, scans, blood tests etc.....