She has to escape.

She has no chance of escaping.

It was bearable, when her owner decided to take a pretty slave girl to bed. She didn't like it, but she lay back and thought of other things and it was endurable. 

But now--now it's not just her on the line. 

Her owner never forbade her to sleep with anyone else, so her claim that the child growing within was probably a full human, given how much less fertile elf/human pairings tended to be, bought her time. But she can't hide it forever--

She runs. She knows she will be found, caught, brought back, but maybe if she's very lucky she can give birth, first, and leave the baby with people who aren't known to have been fucked by an elf, who aren't being watched, maybe she can claim the baby was born dead and they won't bother looking and it can live. 

The snake with a mirror for a face is a surprise.