What are the motifs in Birdsong by Sebastian Faulks?

A recurring idea in Birdsong is the complex romantic relationships in which the characters participate. The most obvious of these is the relationship between Stephen and Isabelle. When they meet, Isabelle is married, but her status as a wife does not prevent Stephen from pursuing her. They engage in an affair and eventually run away together; however, when Isabelle learns that she is pregnant, she worries for the security of her child. Isabelle does not tell Stephen about their child, even when she sees him in Amiens in 1917 during the war, because she fears that it would complicate matters between them.