A pregnancy ultrasound is an imaging test that uses sound waves to see how a fetus is developing in the womb. It is also used to check the female pelvic organs during pregnancy.

You will lie down for the procedure. The person performing the test places a clear, water-based gel on your belly and pelvis area and then moves a hand-held probe over the area. The gel helps the probe transmit sound waves. These waves bounce off the body structures, including the developing fetus, to create a picture on the ultrasound machine.

In some cases, a pregnancy ultrasound may be done by placing the probe into the vagina. For information on this procedure, see transvaginal ultrasound.

A full bladder is necessary to get a good picture. Therefore, you may be asked to drink 2 to 3 glasses of liquid an hour before the test. You should not urinate before the procedure.

There may be some discomfort from pressure on the full bladder. The conducting gel may feel slightly cold and wet. You will not feel the ultrasound waves.

Some doctors order an ultrasound when they think there may be a problem with the pregnancy. Others advocate screening ultrasounds. You should consult your health care provider to determine the most appropriate scanning schedule for you.

Scans may be performed in the first trimester to:

• Confirm a normal pregnancy
• Assess the baby's age
• Rule out problems, such as ectopic pregnancies or potential for miscarriage
• Assess the baby's heart rate
• See if there are multiple pregnancies
• Identify problems of the placenta, uterus, and pelvis

Scans may also be obtained in the second and third trimesters to:

• Assess the baby's age, growth, position, and sometimes gender
• Identify any developmental problems
• Rule out multiple pregnancies
• Evaluate the placenta, amniotic fluid, and pelvis

Some centers are now performing a scan at around 13 - 14 weeks of pregnancy to look for risks of Down syndrome or other developmental abnormalities in the fetus.

The total number of scans will depend on whether a previous scan or blood test has detected problems that require follow-up testing.

The fetus and associated pelvic structures are normal in appearance and appropriate for the gestational age.

Note: Normal results may vary slightly. Talk to your doctor about the meaning of your specific test results.

Abnormal ultrasound results may be due to some of the following conditions:

• Birth defects
• Ectopic pregnancy
• Intrauterine growth retardation
• Multiple pregnancies
• Miscarriage
• Problems with the baby's position in the womb
• Problems with the placenta, including placenta previa and placental abruption
• Too little amniotic fluid
• Too much amniotic fluid ( polyhydramnios)
• Tumors of pregnancy, including gestational trophoblastic disease
• Other problems with the ovaries, uterus, and remaining pelvic structures

There is no documented effect regarding current ultrasound techniques and the risk to women and their developing babies. No ionizing radiation is involved.