San Gabriel Valley Perinatal Medical Group

Your Pregnancy Ultrasound Scan
Please read this carefully

As part of your antenatal care you are having an ultrasound examination (also known as a level II ultrasound scan or sonogram) of your pregnancy. Ultrasound examination of the fetus during pregnancy is generally considered safe when limited to that required to produce the needed information [1]. The examination does not involve x-rays.

Usually the examination  will be through your abdomen and you should have a full bladder. For the examination you will be asked to lie down on an examination table and a clear gel will be applied to your skin over your abdomen . The gel will help to transmit the sound waves generated by the ultrasound probe.  The sound waves that bounce back to the ultrasound probe are used to create pictures on the ultrasound monitor similar to a television screen.

Sometimes the examination will be done through the vagina (transvaginal) to provide a more detailed image, but the doctor will talk to you about this if it proves necessary.

The examination will look for abnormalities in your baby or babies and  will attempt to determine the age and size of your baby or babies. The examination will also look for abnormalities in the placenta. If you do decide to have an ultrasound examination we will assume that you wish to know about anything that we find.
About 60% of major abnormalities will be seen on ultrasound examinations performed between 16 to 20 weeks. Findings suggestive of Down syndrome may be detected about 50% of the time [3,4]. Conditions such as cerebral palsy and autism are not detectable by sonogram before birth. Some malformations of the heart , digestive tract, and face as well as hydrocephalus are most likely to be detected after 26 weeks [2]. In addition to the age of the baby other factors such as maternal obesity, previous abdominal surgery, and the baby's, position may prevent detection of abnormalities.

The table below lists the chances of detecting an abnormality by organ system during a 16 to 20 week sonogram [3].

Organ system Chance of an abnormality being seen
Central nervous system
(brain and spine)
Lungs 78%
(kidneys and bladder)
 69 %
(diaphragm, stomach, esophagus, intestines)
 69 % 
(long bones, feet, and hands)
35 %
Heart 30 to 50 %
(jaw, lip, palate, eye sockets,  and skull)
35 %

If  a problem is found you will be told at the time of the examination that there is a problem. A full discussion of the problem may require you to come back to the office for further evaluation. Some problems that need repeat examination are not serious or are "false alarms".

The examination can sometimes tell what sex the baby appears to be, but not always. If you do not want to know the sex of your baby, please inform the examiner before you begin the examination.


Copyright 2014 by San Gabriel Valley Perinatal Medical Group, Inc. All rights reserved.