Ultrasound: Second trimester examinations (week 14 to 26)
- First Trimester
- Second Trimester
- Third Trimester
- Obstetric Ultrasound Markers
Seeing your baby on a screen is a very exciting and emotional time. Many couples are very eager to see what their baby looks like or to know the sex of their baby but before you have the exam, you should understand that its main purpose is ensure your baby is growing and developing normally.
It is standard practice to perform an ultrasound when the baby is expected to be around 18 weeks old to see if the baby is developing normally. This is known as the anatomy exam.
What is an ultrasound examination?
An ultrasound exam involves transmitting sound waves through the uterus which bounce off the baby and the returning echoes are translated by a computer into an image. Hard tissues are white in the image and soft tissues appear grey and speckled. Fluid (such as the amniotic fluid that the baby lies in) does not reflect any echoes so it appears black. It is the contrast between these different shades of white, grey and black that allows the radiologist to interpret the images.
Is ultrasound safe?
Ultrasound has been used in pregnancy for nearly 30 years and medical research has found no side effects. Even so, most experts agree that the exam shouldn’t be done without clear medical reasons and that all ultrasound exposure should be justified and limited to the minimum needed to make a diagnosis.
Who will do the exam?
The exam is performed by a technologist who is specially trained in ultrasound. After the technologist has taken all the required pictures, a doctor trained in ultrasound (radiologist) will analyze the images and send a report to your family doctor or obstetrician.
Why am I having an ultrasound when the baby is 18 weeks old?
By far the most important part of the exam is to analyze the baby’s anatomy and organs to see if they are developing normally. At this age, the organs have developed to the point where they can now be seen in detail.
In addition to this, the exam also ensures the baby has a heartbeat, determines whether you are pregnant with one or more babies (assuming this is the first ultrasound during the pregnancy), assesses the amount of amniotic fluid around the baby, determines the position of the placenta and ensures that your cervix is closed.
What is examined at the 18 week exam?
Your baby’s internal organs are examined in cross sectional views, or "slices" which may be difficult - if not impossible - for you to make out! Bones will look white on the scan, fluid will be black and soft tissues will look grey and speckled.
This exam involves a detailed evaluation of the fetal head, brain, face, lips, heart, stomach, lungs, abdominal wall, kidneys, bladder, spine arms, legs, hands and feet. Several landmarks within individual organ systems are noted and pictures of these are taken. Only upon request is fetal gender disclosed (assuming it can be seen).
Measurements of your baby’s head, abdominal circumference and thigh bone (femur) are used to give an estimate on its age and to make sure your baby is growing normally. If there is more than seven days’ difference between the measured age and your dates according to your last menstrual period (LMP), the ultrasound dates will be used from now on if this is the first ultrasound during your pregnancy.
What abnormalities can be seen and how effective is ultrasound in detecting them?
This is a difficult question to answer because the technology of ultrasound equipment is advancing so fast that research studies to evaluate the effectiveness cannot catch up with the rapid advancement. Most of the data evaluating the effectiveness for detecting abnormalities dates back to the mid 1990’s. A large study in Europe, the EuroFetus study, was done in the mid 1990’s and the results first became known in 1997. In this study, ultrasound detected 73% of all major abnormalities.
However, this study and others have shown that there is a wide variation in detecting abnormalities depending on whom and under what conditions the examination was performed. Presumably, the expertise, training, time allowed for performing the examination and the quality of the equipment used play a great role in the ability to detect abnormalities. In addition, there has been a massive increase in the technology since the 1990’s which allows centers that have the most up-to-date technology to see the baby in much better resolution. There is no proof that seeing the baby in much higher resolution will pick up more abnormalities, but intuitively it would be expected that machines with better resolution would be better in detecting less obvious abnormalities.
In summary, a quality ultrasound exam should pick up over 75% of major abnormalities, but this means your baby still has a small chance of having an abnormality even if the examination is normal.
Why can it not detect all abnormalities?
Some problems may not be visible until later in your pregnancy. Heart defects, for example, are often not seen at 18 weeks and don’t become visible until much later in the pregnancy or until after birth as the abnormalities often cause a change in the structure of the heart that develops over time and looks like a normal heart earlier in the pregnancy. In other problems (e.g. cerebral palsy, biochemical abnormalities and some chromosomal abnormalities) there is no damage to the structure of the baby that can be seen. The damage is at the microscopic level and it looks normal on ultrasound.
Why couldn’t all the structures of my baby be seen?
Approximately ten per cent of scans will need to be repeated for one reason or another. The most common reason is that the view of the fetus may be hampered by the fetal position at the time of examination. If the baby is lying face down on his head, it is impossible to see the face. If the baby’s back is pointing towards the mother’s front, it can be difficult to see all the organs because the spine is in the way. Also, the tissue interposed between the ultrasound probe and the baby absorbs the ultrasound waves. If there is excess fatty tissue in the mother’s abdominal wall, visualization of the fetus may be difficult.
What if a problem is found?
If a definite abnormality of one of the baby’s body parts or organs is detected or suspected, the radiologist will immediately send a report to your doctor so that you can be sent to a fetal medicine specialist (perinatologist) to verify that an abnormality is present and to discuss of what significance the findings are. Depending on the exact problem, you may be offered genetic testing done from cells of the baby. If the problem does turn out to be serious, you will be given the opportunity to discuss all your options. However, definite structural abnormalities are very rare. Something that is much more common is what we call "soft markers". These findings are not a true abnormality and are very common in completely normal babies, but they may indicate that your baby is at higher risk of having an abnormality. For more information about these, please refer to our information sheet entitled "What are ultrasound markers?"
Why is it important to evaluate my baby’s anatomy before it is 20 weeks old?
In Alberta, if a potential serious but non-lethal defect is suspected, often you will be given the option of having genetic testing on the baby. This needs to be performed before or during the 20th week because it takes up to two weeks to get the results. If a serious abnormality is found, you may be given the option of terminating the pregnancy, but this can only be done up until 22 weeks, six days. After that time period, you cannot terminate the pregnancy even if a serious non-lethal abnormality is discovered after this time period.
Can I learn the sex of my baby?
It is possible to determine the sex from about 18 weeks on, but if your baby is lying in an awkward position it may be difficult to tell. Your technologist will only reveal the sex if you request it; it is not possible to be 100% certain.
What is 4D Ultrasound?
Regular ultrasound only looks at slices of a particular area. These slices are considered two-dimensional and are difficult to interpret without extensive training. New technology has allowed us to obtain multiple images at one time and then a computer reconstructs them into a three dimensional image. The term "4D" refers to an ultrasound machine that can obtain three dimensional images but also display them in real time. With regards to obstetrical ultrasound, this allows you to be able to visualize your baby live in 3D. However, you must realize that this is only used for a short portion of the exam. The majority of the exam is still done in two dimensions, just like any other ultrasound.
Can I have a video of my baby?
If the technologist is able to obtain a satisfactory 4D video of your baby, you will be offered the chance to purchase this on a CD-ROM for a small fee.
This information has been prepared by Dr. Jay A. Zelazo, MD FRCPC (Diagnostic Radiologist) at Canada Diagnostic Centres.
