From my experience of being pregnant and also from hearing my friends' experiences, I can assure you that for every woman it can be completely different.

Some women NEVER want sex the whole of pregnancy, I think it has to do with a protective instinct, however misguided this could be in a normal pregnancy. They can't help it. Some ppl's sex drive seems to go through the roof, others want it once a trimester...

I don't think there is a definitive answer to why this could be happening, but may I recommend lots of cuddles, kisses and 'i love you's, 'you look gorgeous today' etc - it may make her feel like it's time.

Don't worry, things should eventually return to normal. Good luck.