It's usually nothing. Most of the time the cause is unknown. Sometimes it resolves itself and sometimes you just have a lot of fluid until you deliver. Also, ultrasounds are bad at estimating it. They've been known to be inaccurate.
In the interest of full disclosure, high amniotic fluid is found is pregnancies with problems, like diabetes, birth defects (minor and major) etc. But most of the time it's nothing. So don't worry yourself :)
at 1:53 PM on Oct. 30, 2009