Hello
I only registered to give you this answer since I was looking for it too and wanted to let you know I was successful. It was hard to find. No one seemed to know it but I knew it was not Brahm's lullaby.
It was composed by Alexandre Desplat for the film Birth (starring Nicole Kidman) the track is called The Engagement (track 2). It is a leitmotif throughout the album (particularly prevalent in the last track).
The only way I found it was to randomly select composers who I thought sounded similar (starting with Philip Glass) and hoped that the "listeners also bought" feature on itunes worked to bring it up eventually. It did (though I think some luck was involved since I only had to listen to 2 albums before I found it). It is a great track but the whole album is brilliant.
Enjoy!