18-20 week screening pregnancy ultrasound
Dr Monica Pahuja
Date last modified: May 01, 2009
1. What is an 18-20 week screening pregnancy ultrasound?
The 18-20 week ultrasound is considered part of your routine care during your pregnancy. Your referring doctor will discuss this with you. The examination is performed at this stage in your pregnancy because the foetus (your unborn child) is big enough to assess foetal anatomy and normal development.
It also provides additional information such as:
- The number of foetuses
- The age of the foetus
- Confirmation of your due date
- The position of the placenta (the organ that supplies nutrients and oxygen to the foetus through the umbilical cord)
- The volume of the fluid around the foetus
- The appearance of your cervix (the neck of your uterus).
“Ultrasound” is the term used for high frequency soundwaves (see Ultrasound). It can be performed while there is movement so is ideal for imaging the foetus.
2. How do I prepare for an 18-20 week screening pregnancy ultrasound?
There is very little preparation needed. You should have some fluid in the bladder but not be uncomfortably full. Drinking water prior to the examination will enlarge the bladder, enabling the surrounding internal areas to be examined. Staff where you are having the ultrasound will provide you with information about this.
It is a good idea to wear comfortable clothing that gives easy access to your entire abdomen.
Some research has found that when children are present during this examination in the ultrasound room they can become restless, which can distract the person performing the examination and some important aspects of the screening may be missed.
It is strongly recommended that if you have children, you arrange for child care beforehand and do not bring children to your scan appointment if there is no one to care for them in the waiting room.
3. What happens during an 18-20 week screening pregnancy ultrasound?
You are asked to lie on an examination couch. The abdomen is exposed and water based clear gel is applied to the skin.
An ultrasound examination is performed by a trained health professional (a sonographer, radiologist, or obstetrician sonologist) using a smooth hand held device called a transducer that is moved gently across the abdomen (see Ultrasound).
The transducer transmits ultrasound waves through the abdomen which are relayed back to the ultrasound machine to produce images or pictures of the foetus which you will see on the ultrasound screen.
The examination is performed in real time. This means that the images you see on the screen show what is happening inside your uterus at that moment like a movie. Pictures or images are taken during the examination.
The experience of seeing your unborn baby is exciting and positive and the sonographer will generally point out easily recognised parts of the body as the scan is being performed. You may not recognise or understand some of the images you see on the ultrasound screen, but it is all part of the important and thorough examination being performed.
Many people wish to know the sex of the baby as it can usually be seen at this time. If you would like to know, ask the sonographer to tell you. Mostly it is straightforward to determine sex of the baby, but occasionally the sonographer will not be able to tell and/or position of baby is such that the genitalia are not well seen. If it is not possible to tell the sex of the baby you will not receive another ultrasound for that purpose. You should also be aware that assessment of gender is not 100%. If you do not want to know tell the sonographer before having the ultrasound scan.
The ultrasound is performed for medical reasons to fully check and assess the development of the foetus from head to toe.
A number of measurements of the foetus will also be taken during the scan (head size, abdomen and bones) to asses the exact size of the foetus. The position of the placenta will be determined to assess whether it is away from the cervix. The cervix will also be assessed to make sure it is closed. If the cervix is short or open there is a risk of early labour or miscarriage.
Sometimes the foetus may not be in an ideal position to see a particular part of the anatomy, and the sonographer may ask you to change position slightly by rolling from one side to the other. Occasionally the foetus is in such an awkward position that the anatomy cannot be seen, so you may be asked to return on another day to complete the procedure. This should not alarm you and is a common occurrence.
If this occurs and things can't be seen clearly from a trans-abdominal scan, or it is necessary to check if the placenta is close to the cervix, a transvaginal ultrasound may need to be performed (see Transvaginal Ultrasound) where a different shape of transducer is inserted into the vagina. This transducer is slightly larger than a tampon and shaped to fit comfortably into the vagina. If a Transvaginal ultrasound is necessary to perform an examination of the foetus, the sonographer, sonologist, or specialist doctor will fully explain what is involved and obtain your consent before doing this type of study.
The person performing the examination will be concentrating very closely on the images as they come onto the screen. Foetal ultrasound is a complex examination as there are hundreds of structures in the developing foetus that need to be checked and measured. They will have a well established routine for doing this and a checklist so that nothing is overlooked.
A normal foetus will be moving quite a bit during the scan, and it may take a few minutes to get exactly the right picture of a hand, foot, the brain, or various parts of the foetal chest or abdomen. Do not be concerned if the sonographer, sonologist or specialist doctor is quiet while performing the scan as they are concentrating on the complex examination.
4. Are there any after effects of an 18-20 week screening pregnancy ultrasound?
There are no after effects of an 18-20 week screening pregnancy ultrasound.
5. How long does an 18-20 week screening pregnancy ultrasound take?
The scan usually takes between 30-45 minutes.
6. What are the risks of an 18-20 week screening pregnancy ultrasound?
There are no risks associated with this ultrasound scan. Ultrasound uses high frequency soundwaves to produce an image onto a screen and does not use ionising radiation, which means that it is very safe. Ultrasound has been in use since 1950. There is no evidence to show any biological or hazardous effects to you or your foetus.
Not all abnormalities can be seen on ultrasound. Ultrasound accuracy is about 60% for detecting abnormalities. About 50% of Down syndrome (one of the most common chromosomal abnormalities) are detected by ultrasound. In addition, some conditions such as autism, intellectual disability, cerebral palsy or rare chromosomal abnormalities are not identified as an abnormal finding on ultrasound. A normal ultrasound result for a foetus does not necessarily mean normal development will continue throughout infancy.
The examination is difficult in some patients who are obese as the quality of the pictures are often not as clear because the foetus is further away from the ultrasound transducer.
7. What are the benefits of an 18-20 week screening pregnancy ultrasound?
The 18-20 week screening pregnancy ultrasound is a screening test to examine the development of the foetus. A foetus has normal development in 98-99% of pregnancies. A small number of 18-20 week screening pregnancy ultrasound scans (1-2%) will identify a major structural abnormality.
The significance of any abnormality will be explained to you by a doctor and it may be necessary to have further ultrasounds at specialist centres dedicated to scanning foetuses and abnormalities.
You may need to have a “needle test” such as amniocentesis (see Amniocentesis) or chorionic villus sampling (see Chorion villus sampling) to check the chromosomes of the foetus if Down Syndrome or other congenital abnormalities are suspected.
If abnormalities are detected, it is the role of the radiologist or obstetrician sonologist to talk with your doctor about the possible causes for the abnormalities seen on the ultrasound scan. This will help to guide the discussion between you and your doctor about any further investigation or treatment that may be needed.
8. Who does the 18-20 week screening pregnancy ultrasound?
The examination is performed by sonographers who are health professionals specially trained and accredited to perform the test. Sonographers may be male or female. If you are not comfortable with a male you should let the reception staff know this prior to having the test.
The sonographer may leave the room to show the pictures to the reporting doctor who may come in and scan again if something needs to be checked.
At some hospitals or radiology practices a doctor or specialist may perform the examination. This specialist may be a radiologist or an obstetrician who has specialised in pregnancy and women’s ultrasound.
9. Where is an 18-20 week screening pregnancy ultrasound done?
The examination is performed in a radiology department of a hospital, private radiology practice or at a specialist clinic for obstetric and gynaecological imaging. The examination is performed in the privacy of an ultrasound room which may be dimly lit to allow the images on the ultrasound machine to be clearly seen by the person performing the scanning.
10. When can I expect the results of my 18-20 week screening pregnancy ultrasound?
The time that it takes your doctor to receive a written report on the test or procedure you have had will vary, depending on:
- the urgency with which the result is needed
- the complexity of the examination
- whether more information is needed from your doctor before the examination can be interpreted by the radiologist
- whether you have had previous X-rays or other medical imaging that needs to be compared with this new test or procedure (this is commonly the case if you have a disease or condition that is being followed to assess your progress)
- how the report is conveyed from the practice or hospital to your doctor (in other words, email, fax or mail)
Please feel free to ask the private practice, clinic, or hospital where you are having your test or procedure when your doctor is likely to have the written report.
It is important that you discuss the results with the doctor who referred you, either in person or on the telephone, so that they can explain what the results mean for you.