Ultrasound imaging or sonography is based on the priciple of piezo-electric effect.Sound waves are produced by a piezoelectric crystal and images are produced by sound waves being reflected by various organs in the body.It does not use any ionizing radiation.
 
 
It is used in pregnancy to determine following things-Number of fetuses, lie, presentation, fetal maturity, placental localiztion, liquor and congenital anomalies.
 
 
Evaluation of early preganancy requires a full bladder (oops!!!) and it is not essential for later stages of pregnancy. Nowadays even three dimensional and four dimensional (3D in real time) are also available which can give 3D representations of fetal face, spine and various congenital anomalies and give the parent an oppurtunity to decide after seeing the baby.

Doppler US can also be used especially in high risk mother like Pregnancy induced hypertension and IUGR to assess utero-placental and fetal blood flow dynamics.

Please note-ultrasound cannot identify all fetal abnormalities. It can determine the sex of the baby (but telling the sex of the baby is illegal in India!!)
 
Reference-
Information For The Patient
http://www.radiologyinformation.blogspot.com
 
Sumer's Radiology Site
http://www.sumerdoc.blogspot.com