What is an obstetrical ultrasound?
Obstetrical ultrasounds or fetal ultrasounds are an extremely important diagnostic imaging test used during a woman’s pregnancy. This exam is critical in examining the uterus and placenta, as well as the fetus in its entirety. Early obstetrical ultrasounds focus on the location, size, and number of pregnancies, whereas later scans will focus on the development and anatomy of the fetus and the other components of the pregnancy. Ultrasound utilizes a transducer to send and receive sound waves, and this information is used to create a real time image. As a result of this ability, and the safe application of sound waves (ultrasound), ultrasound is the leading diagnostic tool for obstetrics.
First trimester (up to 14 weeks) ultrasound:
During the first trimester there may be 1-2 ultrasounds ordered routinely. This is done to:
- Confirm your estimated due date more accurately by measuring the embryo (after the first trimester, ultrasound measurements of the fetus to date the pregnancy are less accurate)
- Confirm the fetal heartbeat
- Make sure the pregnancy is located within the uterus.
- Determine the number of embryos (twin gestation, or other multiples.)
At Beam Radiology we have developed a screening model for aneuploidy detection, specifically T21, T18, and T13, with a higher sensitivity and specificity than the current first trimester screen offered in Calgary. This utilizes both an in-depth ultrasound of the early fetal anatomy (around 12 weeks gestation) and bloodwork analysis called the non-Invasive prenatal screen (available as early as 10 weeks gestation) for aneuploidy detection. These two components combined constitute a new first trimester screen, known as the advanced first trimester screen, or aFTS.
This initiative is available to all pregnant patients.
Currently in Alberta the standard is that between 11 weeks and 13 weeks 6 days gestation there is the opportunity to receive aneuploidy screening. This type of ultrasound may be done along with a blood test to determine your baby’s risk of having Trisomy 21 (Down syndrome), Trisomy 18 (Edward’s syndrome), and Trisomy 13 (Pateau’s Syndrome). There is also an ability to screen for the presence of a neural tube defect. This is called the combined first trimester screen and utilizes bloodwork in combination with an ultrasound that measures the nuchal translucency of the fetus, this is a less sensitive screen than the Beam aFTS.
It is also possible to receive screening based only on maternal age, and adjusted following an ultrasound to ascertain the nuchal translucency, with no bloodwork component. This is known as the nuchal translucency ultrasound.
Of course, if the aneuploidy risk assessment component for is not desired, there is still an opportunity to assess the condition of the fetus at this stage and assess the first trimester anatomy by ultrasound alone.
Click here to learn more about the Advanced First Trimester Screen, the aFTS, and how to access this tool with your doctor.
Second trimester (15 to 27 weeks) ultrasound:
The second trimester takes place from the 15th week to the 27th week of gestation. In this trimester there is a routine ultrasound done at 18-22 weeks gestation called the Detailed Anatomical Scan. Ideally, we will see you between 19-20 weeks. This is the best time to observe the fetal anatomy and detect possible congenital defects or differences (birth defects). At this ultrasound it is also possible to look for “markers.” Markers are not abnormalities, but rather signs that can indicate a more serious condition. In most cases a marker is simply a normal variant. With this information your care provider can offer further testing or recommend follow up.
Third trimester (after 27 weeks) ultrasound:
Your health care provider may ask you to have one or more third trimester ultrasounds for many different reasons. Some of these reasons may include:
- Determining your baby’s size, growth, and position
- Determining the amount of amniotic fluid surrounding your baby
- Determining the location of your placenta
- Measuring the length of your cervix
- A woman’s current or past health conditions that may affect the pregnancy
- A woman’s obstetrical history
Frequently asked questions:
In general, an obstetrical ultrasound can:
- Determine the pregnancy site, and the number of embryos. (Twins etc.)
- Look at the size of an early pregnancy to provide a gestational age and estimated due date.
- At the appropriate gestational age, it can look to see if your baby is developing normally or if there are any major anatomical concerns.
- It can assess and track the size and growth of the fetus.
- Later in pregnancy it can assess the amniotic fluid volume.
- Later in pregnancy it can show what position your baby is in.
- Help investigate any concerns your health care provider might have about your uterus, ovaries, placenta, or cervix.
Obstetrical Ultrasounds can be performed in two ways:
- Transabdominal Ultrasound – The transducer is moved over your abdomen and pelvis.
- Endovaginal Ultrasound – An endovaginal transducer with a sterile probe cover is inserted into the vagina. Depending on the stage of pregnancy this can offer a view of the entire uterus or a limited assessment. This method is only used with the consent of the patient and is safe for pregnancy.
Factors that might limit the amount of information received throughout your pregnancy include:
- Ultrasound waves weaken as they travel, so if they are travelling further the signal will be weaker, and the image will not be as clear. Therefore, if a patient is larger in size, or obese, the resolution of the image will be lower.
- The position of the baby will impact what is visible to be assessed.
- The size of the baby
- The amount of amniotic fluid present
- The activity level of the fetus. If there is an excess of movement it can be more challenging to get clear imaging.
- If you are having your ultrasound in the first or second trimester and arrive with an empty bladder there may be difficulty in properly visualizing the pregnancy.
Empty your bladder 90 minutes before exam time, then consume one 8-ounce glasses of fluid (water, juice, etc.) about an hour before exam. We recommend a two-piece outfit so we can access your pelvis without you removing your clothing. You may eat normally prior to having an obstetrical ultrasound.
For a third trimester ultrasound (28-42 weeks), you are asked to eat prior to your exam.
Please consider the length of the appointment and any decision to bring small children.
You may have one support person with you during your medical ultrasound. This is only the case with obstetrical ultrasounds.
** In consideration of COVID-19 this guidance may be different or altered to comply with health and safety standards of the time.
What to expect at your obstetrical ultrasound is dependent on the exam:
- Transabdominal ultrasound: A transabdominal approach to the fetal ultrasound is the most utilized. This is done by moving the transducer over your pelvis. During a transabdominal fetal ultrasound, you will be invited to lay down on the ultrasound bed, in a semi-reclined position, and expose your abdomen and pelvis.
- Endovaginal ultrasound: If you are having an endovaginal ultrasound you will be asked to change into a gown or undress from the waist down. You will lay down on the exam table with a sheet covering your lower body. The transducer will be covered by an endovaginal probe cover and sterile gel will be applied to the outside of the cover. The technologist will pass you the transducer, while keeping you covered, and you will be able to insert it yourself. Nothing about this process should be painful or uncomfortable for you so you will be encouraged to communicate with the sonographer if anything does bother you at all. Endovaginal ultrasounds are used most often during early pregnancy, or if later on, to assess the cervical length.
Obstetrical ultrasound has been used in pregnancy for over 50 years. Many studies have been completed to determine if an obstetrical ultrasound is safe. Ultrasound is used around the world as a safe modality for evaluating the health of the embryo and fetus.
All obstetrical ultrasounds are ordered by your doctor, midwife, or nurse practitioner. They must be done by trained and accredited medical professionals called sonographers. Sonographers have special knowledge and skill in performing obstetrical ultrasounds.
***For more information about specific obstetrical ultrasound exams please see our blog posts regarding the Dating Ultrasound, The Anatomical Scan, and the Third Trimester Ultrasound***