Helpful Links:

Obstetric Ultrasound from RadiologyInfo.

Obstetric ultrasound -- a comprehensive guide to ultrasound scans in pregnancy.
American Institute of Ultrasound in Medicine.
International Society of Ultrasound in Obstetrics and Gynecology.
Ultrasound in obstetrics and gynecology: Guidelines, statements, opinions.
Pregnancy ultrasound from MEDLINEplus
Prenatal Ultrasound Links from OMNI.
Ultrasonographic "soft markers" of fetal chromosomal defects.
Soft Markers - A Guide for Professionals.
Fetal Soft Markers in Obstetric Ultrasound.
First trimester Down's screening from GeneCare.
The RCOG paper on routine screening in pregnancy.
Safety considerations from the British Medical Ultrasound Society.
Worrying concerns from Birth.com.au.
On the Safety and Usefulness of Prenatal ultrasound.
Ultrasound, more harm than good?.
The fetus hears the ultrasound?.
A set of articles on the accuracy, safety and pitfalls of prenatal ultrasound by mom.
Obstetic ultrasonography from Answers.com.
Practice Guidelines of the Society of Obstetricians and Gynaecologists of Canada.
Dr. Greg Devore's Prenatal Diagnosis Homepage.
Fetal Ultrasound Online (Peter W. Callen).
3D Ultrasound in Obstetrics and Gynecology.
Dr. Najeeb Layyous's 3-D and 4-D pictures.
3-D ultrasound, University of California -- San Diego.
University of Cambridge 3D ultrasound Research.
University College London, 3D Fetal studies.
OBGYN.net Ultrasound.
Safety of 3D ultrasound from AskQuestions.org.
IBIS Birth Defects Information System.
Dr. Hutchon's Obstetric calculator page.
The Gestation Network.
Fetal Medicine Unit, St. George's Hospital Medical School, London.
Fetal surgery at UCSF.
The Fetus.net from Dr. Philippe Jeanty.
SonoWorld.
Michael Applebaum's Ob/Gyn ultrasound site.
Perinatology network -- Ultrasound.
The AmnioNet.
ObGyn Ultrasound at Fairbanks Clinic.