2D, 3D and 4D Obstetric Ultrasound
Early first trimester ultrasound
Not all pregnant women need an early first trimester ultrasound. Some of the reasons why an early first trimester ultrasound may be requested are listed below:
- Uncertain last period
- Bleeding
- Pain
- Past history of ectopic pregnancy
- Excessive nausea
- Recurrent miscarriages
On a first trimester ultrasound we can:
- Confirm that the embryo is alive by seeing heart movements. These should be seen from 6 weeks onwards, when the embryo is 5mm.
- Calculate the gestation and the estimated due date
- Diagnose multiple pregnancy
- Diagnose ectopic pregnancy (pregnancy outside the uterus)
- Check the uterus and ovaries for fibroids or cysts.
Early in pregnancy transabdominal views may be very poor. It may therefore be advisable to perform a transvaginal ultrasound. A narrow, gel covered probe is gently introduced into the vagina. Most patients experience no pain or only very mild discomfort. The ultrasound is performed with an empty bladder and can be performed even when there is bleeding.
The 11-14 week nuchal translucency ultrasound
This ultrasound can be offered for Down Syndrome screening. More information regarding this ultrasound can be found under 'Down Syndrome Screening'
The Routine 18-20 Week Ultrasound Examination
Most pregnant women will be offered a routine midtrimester ultrasound examination. It can be performed from 18 weeks onwards but is preferably done between 19 and 20 weeks. The ultrasound is performed transabdominally. A full bladder is not required.
The purpose of this ultrasound is:
- To confirm that the fetus is alive.
- To diagnose multiple pregnancy if no earlier scan was performed
- To confirm your due date if no earlier scan was performed
- To assess fetal growth by measuring the fetal head, abdomen and femur
- To assess the fetal anatomy. This involves a detailed examination of the fetal head, brain, face, lips, heart, stomach, lungs, abdominal wall, kidneys, bladder, spine, arms, legs, hands, and feet. Upon request, the fetal gender can be disclosed in most situations.
- To assess the position of the placenta. In approximately 3%of pregnancies the placenta is low-lying at the midtrimester ultrasound. A repeat ultrasound is then recommended around 32 weeks. In the majority of patients, the placenta will no longer be low-lying then.
- To assess the liquor volume.
- To assess the length of the cervix.
- To check for ovarian cysts or fibroids.
This examination is expected to detect the majority of major fetal malformations. It is important to appreciate however that even with the best ultrasound equipment not all abnormalities can be seen.
Sometimes abnormalities can be missed because suboptimal views are obtained. This can be due to difficult fetal position or the patient's weight. In these situations a repeat ultrasound is often organised a few weeks later when the fetus is bigger and/or in a different, hopefully better position.
Other times abnormalities may be missed despite good views. This may happen with conditions where the abnormality only becomes evident in later pregnancy or where there are in fact no structural changes in the fetus.
The third trimester ultrasound
Not all pregnant women need a third trimester ultrasound. Some of the reasons why a third trimester ultrasound may be requested are listed below:
- If clinically the fetus feels larger or smaller than expected for the gestation
- If the mother has a medical condition that may influence fetal growth and well-being
- If the placenta was low-lying on the midtrimester ultrasound.
- If the clinician is uncertain about the position of the fetal head
- If there is pain of bleeding
- If the fetus is not moving well
- If a fetal abnormality was noted on an earlier ultrasound
On a third trimester ultrasound we always check:
- Fetal anatomy, although late in pregnancy the views of certain parts of the fetus may be difficult.
- Fetal position.
- Placental position
- Fetal size. An estimated fetal weight is calculated from measuring the fetal head, abdomen and femur. There can be a 15% error on the weight estimate.
- Fetal well-being. We check fetal movement, tone, breathing and liquor volume and give the fetus a biophysical score which is indicative of its well-being. The maximum score on ultrasound is 8/8.
- The blood flow in the umbilical cord, which gives an idea of placental function
Fetal assessment in 3D or 4D
In conventional 2D scanning only a thin slice of the fetus can be seen at any one time. The anatomy of the fetus is checked by looking at consecutive slices through the whole body. Although the image is very informative the picture does often not look like a baby. With 3D ultrasound a whole series of slices is taken and digitally reconstructed into a 3D image to produce life-like pictures of the fetus. 4D ultrasound just adds the element of time to the process which means that these life-like pictures can be seen to move in real time.
There is no doubt that when good views are obtained parents-to-be enjoy the 3D image of their baby as it moves in real-time. It must be said however that the wider benefits of 3D ultrasound in pregnancy are yet to be determined.
For the assessment of the fetal anatomy most sonologists still rely almost solely on 2D ultrasound and 3D or 4D ultrasound is switched on at the end of the examination for fun.
At Camberwell Ultrasound for Women and Central Ultrasound for Women, we do our best under all circumstances to get good 3D images. However, there are several factors outside of our control which will determine the quality of the images.
- Fetal position: Most people want to see their baby's face but we need a good position and a lot of fluid around baby's face. Some babies turn their back on us. Others cuddle up to the placenta or uterine wall, or hide behind their hands (and feet) making 3D and 4D imaging difficult.
- Maternal tissue: The more tissue the sound waves need to travel through, the less contrast the resulting images have.
- Fetal activity: It is fun to see the baby in motion with 4D ultrasound. On the other hand, the best still 3-D photo results come when baby is asleep or still.
- Gestational development: The best 3D images are obtained around 28 weeks. The baby is bigger and there is a lot of fluid around the baby, but we try to get the best possible images at the midtrimester scan since most women don't need a 28 week scan.
We encourage patients to enjoy the fun but not to expect the glamour shot to avoid disappointment.
