Yes, it is foreshadowed.
It is explicitely said , in book 7, that Bellatrix talks to Voldemort "as a lover". Also, he is suspiciously furious when she dies.
The possibility of Bellatrix having a child is briefly evoked in book 6. When Narcissa is worried about Draco's mission ans asks Snape to help him, Bellatrix says she should proud that Draco was chosen and adds that, if she had sons, she would want them to be Voldemort's servants. 
Bellatrix was probably already pregnant at this time and was thinking about her child's future. (The child turned out to be a girl but, of course, Bellatrix could not know this yet.)