The heartbeat is a good sign. The lack of amniotic fluid could account for the lack of fetal movement. It's not unusualy for 2nd and 3rd trimester ultrasounds to be off by a week or two either way.
The doctor should closely monitor the baby and the fluid levels frequently. My first pregnancy suffered from oligohydrmniosis. I made it to 40 weeks but ended up wtih a c-section due to the fact that she suffered from growth restriction, she was breech and I had NO fluid left.
Like KMZ said, your wife needs to stay well hydrated and take it easy.
Oligohydramnios basically means that your wife's amniotic fluid is low.
It is believed that simply keeping the mother as hydrated as possible may help to increase the volume of the amniotic fluid, however it is not medically proven.
Unfortunately, complications such as cord compression can arise however if it is monitored properly and your wife is taking it easy, there is a very good chance that you're baby will be just fine.
But, it comes to your baby's birth, low amniotic fluid can complicate things. There is a posibiltity that the baby may be in breech position as it may not have the room to get into a head down position and engage.
Obviously, an OB will be able to confirm the position of your baby closer to the date of delivery and assist in deciding how the baby should be born.
Good luck :)