White foam in stool in 36 week of prgnancy

No idea. Have you talked to your doctor about this? It is hard to say what is causing this, especially online. It is much safer to ask your doctor. They're used to this kind of question.

Are you taking any medications? Is it possible that what you are seeing is mucus? That would not be uncommon, but as CravenMorhead said, you should check with your doctor if you are seeing it more than once to discuss possible reasons; some of which will need to be addressed.