Ultrasound scan—your 18 to 20 week scan
Your 18 to 20 week ultrasound scan is also known as the morphology scan or fetal anomaly scan. It looks for abnormalities in your baby's structural development and growth. It also checks the position of the placenta. It is not a screening test for chromosomal anomalies. The ultrasound scan also provides you with an opportunity to see your baby, often for the first time. While this can be a very exciting and emotional time it is important to remember that this is a medical examination and the main purpose of the scan is to detect structural abnormalities.
The ultrasound scan
The ultrasound scan is performed by a sonographer who is specially trained to assess your baby. The sonographer will place some ultrasound gel on your abdomen before using the transducer against your skin to create images of your baby.
The ultrasound works by pulses of sound waves passing from the transducer into the uterus which then bounce harmlessly off your baby, creating echoes. Your baby is unable to hear the sound as the power of the sound is very low. The computer then changes these echoes into images which appear on the screen and may be difficult for you to interpret. Bones appear white, fluid black and tissue a grey speckled colour.
Please do not be alarmed if a sonographer re-examines a part of your baby or asks a colleague to be in attendance during your ultrasound. Often one of the specialist doctors in the department may come in to scan your baby. This may be necessary in order to get the most information from the morphology scan of your baby.
If you would like a picture of the scan, please let the sonographer know at the start of the scan. The use of cameras or video equipment is not permitted.
Areas examined during the ultrasound scan
The head and its internal structures are examined closely. Measurements including the head circumference (HC) and biparietal diameter (BPD) are recorded and your baby's face is examined to see if there is any cleft of the lip. Clefts of the palate are very difficult to examine and often not detected by ultrasound.
The spine is checked to make sure that it is fully covered by skin and that the vertebra are forming and in alignment.
The abdominal wall is checked to ensure that it encases all the internal organs and the abdominal circumference is measured to assist with calculating your baby's size.
The heart is examined to see if the four chambers are the appropriate size and connected by valves which can be seen opening and closing with each heart beat. The main vessels connecting the heart are examined. If there are concerns about your baby's heart it will usually be re-examined at around 24 weeks when the connecting vessels can be more clearly visualised.
The stomach should be located just below the heart and be filled with some amniotic fluid that your baby is constantly swallowing.
The kidneys and bladder will be checked to see if they are formed and functioning properly.
The arms, legs, hands and feet are checked to see if they are present. The femur (leg) and humerus (arm) bones are measured to check they are growing appropriately.
The placenta's position in the uterus is recorded. If it is close to the cervix the distance will be measured and a repeat scan maybe recommended between 32 and 34 weeks to check that the placenta has moved away from the cervix. Most placentas will have moved away from the cervix by this time.
The umbilical cord will be examined. The number of vessels within the cord is counted. Normally there are two arteries and one vein.
The amniotic fluid is assessed to check that the amount is within the normal range.
Can I find out whether my baby is a boy or girl?
If there is good visualisation of your baby and if you wish to know, the sonographer can notify you of the likely sex of your baby.
Limitations of the scan
Although a number of birth defects can be identified by ultrasound, there are many that will not be detected. At 18 to 20 weeks the detection rates for structural anomalies is 40 to 70 per cent. It is important to remember that a normal ultrasound does not always mean your baby will be born without any abnormalities.
Generally the earliest time to look for birth defects is between 18 and 20 weeks of pregnancy however, some will not become evident until late in the pregnancy. Anatomic areas such as the heart, the face and the hands are difficult to assess and not all defects are detectable.
Other factors such as your build, scars from a previous operation and the way your baby is positioned may limit the diagnostic ability of this test.
At the Mater Centre for Maternal Fetal Medicine you will be given the results on the day of your scan and the findings will be explained to you. A copy of the report will be sent to your referring doctor. If an abnormality is detected, a maternal fetal medicine specialist will counsel you regarding what it may mean; whether further tests are required to confirm the diagnosis and what treatment options are available.
Not all services provided by the centre are covered by Medicare or your private health insurance. A gap payment will be incurred for most services. Bulk billing is available for women with pension or health care cards who present their cards on the day of their service. If you have any queries regarding fees, please discuss them with our staff prior to your scan by telephoning 07 3163 1896.
Booking your 18 to 20 week ultrasound scan
As a tertiary referral centre, we have limited appointments for routine ultrasounds. Therefore, we recommend that your doctor faxes a referral as soon as possible via 07 3163 1890.
Preparation for your scan
Please eat and drink normally as a full bladder is not required for your ultrasound scan. However, we request that you do not empty your bladder within 30 minutes of your appointment, unless you are uncomfortable, as some fluid in the bladder improves visualisation.
Please phone 07 3163 1896 if you are unable to attend your appointment for any reason. If you arrive late for your appointment, it may need to be rescheduled.
Centre for Maternal Fetal Medicine
Mater Mothers' Hospital
South Brisbane Qld 4101
Phone: 07 3163 1896
Fax: 07 3163 1890
Mater acknowledges consumer consultation in the development of this patient information.
Last modified 06/11/2015.