Pregnancy ultrasounds are performed mainly using transabdominal ultrasound.
For many women, especially after 8 weeks gestation, sufficient information about the baby may be obtained with transabdominal ultrasound only. However, in the early pregnancy, the developing embryo is very small (at 6 weeks gestation, the baby is only 5-9mm long) and a transvaginal ultrasound may be required to get a better image of the baby. Transvaginal ultrasound is safe and commonly performed during all stages of pregnancy, including the first trimester. It will not harm you or your baby. Transabdominal ultrasound involves scanning through your lower abdomen. A small amount of ultrasound gel is put on the skin of the lower abdomen, with the ultrasound probe then scanning through this gel. The gel helps improve contact between the probe and your skin. Transvaginal ultrasound is an internal ultrasound. It involves scanning with the ultrasound probe lying in the vagina. Transvaginal ultrasound usually produces better and clearer images of the female pelvic organs including the developing pregnancy, because the ultrasound probe lies closer to these structures. The transvaginal ultrasound probe is thin, about 2cm diameter. The probe is covered with a disposable protective sheath. A small amount of ultrasound gel is placed on the end of this probe. The probe is then gently inserted a short distance into the vagina. All transvaginal probes have been cleaned and sterilized according to recommended protocols. Performing the transvaginal ultrasound usually causes less discomfort than a pap smear. No analgesia is required for this ultrasound. Your privacy will always be respected during your ultrasound, especially the transvaginal examination. You will have a large towel covering your lower body, in addition to wearing a gown during the transvaginal ultrasound.
A first trimester ultrasound will usually include each of the following components however some ultrasounds may focus more on particular areas. Your ultrasound is always performed in the context of your clinical history and the results of previous ultrasounds and investigations.
The nuchal translucency is the fluid found at the back of your baby’s head and neck, just beneath the skin. The thickness of this fluid can be precisely measured and this is called the nuchal translucency (or NT) measurement. Normally the amount of fluid is small, producing a thin NT measurement. We know that the amount of fluid can increase in the presence of certain conditions, producing a thicker NT measurement. Conditions associated with an increased NT measurement include some chromosome abnormalities (for example, trisomy 13, 18 and 21) and some structural problems (for example, heart abnormalities). An increased NT measurement does not always mean the baby has a problem but it does increase the risk. There are established and strict criteria for the accurate measurement of the nuchal translucency. It is important for the measurement to be done properly to ensure an accurate result. If your baby is not co-operating and the NT cannot be measured accurately, we may ask you to return for more images later in the day. Occasionally, patients need to return on another day if the baby remains in an unhelpful position.
Prenatal diagnostic testing is used to determine whether your baby has a chromosomal abnormality. The most common types of prenatal diagnostic tests available are chorionic villus sampling (often simply called “CVS”) and amniocentesis (often simply called “amnio”). Chorionic villus sampling and amniocentesis are different from prenatal screening tests such as nuchal translucency scans. Prenatal diagnostic testing is considered an invasive way of determining the presence or absence of a chromosome abnormality. These tests involve putting a needle into either the placenta or amniotic sac of the developing pregnancy and hence they are associated with an increased risk of miscarriage. These tests are usually performed only in certain situations.
Chorionic villus sampling is a prenatal diagnostic test, usually performed between 10-13 weeks gestation. The test involves obtaining a small amount of placental tissue from the developing pregnancy. The placenta contains chorionic villi. Chorionic villi usually have the same genetic (chromosomal) makeup as the baby. The chorionic villi are grown in the laboratory and the chromosomes examined.
Not all women need prenatal diagnostic testing, including chorionic villus sampling. Chorionic villus sampling may be suggested for a number of reasons including:
A second trimester morphology ultrasound is performed in the middle part of the pregnancy, usually between 18 and 20 weeks gestation. All pregnant women have this ultrasound as a routine part of their antenatal (pregnancy) care. This ultrasound will check that your baby is growing and developing normally.
A third trimester ultrasound is performed in the last part of the pregnancy, usually after 22-24 weeks gestation. Not all women need to have an ultrasound in the later part of the pregnancy. Your doctor may request a third trimester ultrasound for many reasons including:
Most ultrasounds are routinely performed using conventional 2 dimensional (2D) scanning. This is where the ultrasound beam scans through a thin slice of the woman’s body or the baby, and shows the image on a screen in 2 dimensions only. The image usually appears grey on the ultrasound screen, although the sonographer may sometimes change the color of the picture. This type of ultrasound provides most of the clinically relevant information to the doctor.
In recent years, 3 dimensional (3D) and 4 dimensional (4D) ultrasounds have become increasingly popular and readily available. With 3D ultrasound, a series of thin 2D slices is digitally reconstructed to give more life-like images. With 4D ultrasound, the added dimension is time, so that the 3D images appear to be moving in real time. The 3D/4D image usually appears a golden color on the ultrasound screen, as this color is easy for patients to look at and highlights features on the baby.
3D/4D ultrasound uses the same ultrasound beams as the conventional 2D ultrasound, with extra processing performed by the ultrasound machine computer. 3D/4D ultrasound is just as safe as conventional 2D scanning, with no evidence that it harms you or your baby. There is some diagnostic role for 3D/4D ultrasound in obstetrics, although this is currently limited to a few particular conditions, example diagnosing cleft lip/palate.
Most 3D/4D images in pregnancy are taken for the enjoyment of parents, rather than for their clinical usefulness. It is a good idea for parents to remember the limitations of 3D/4D when they are having their ultrasound.
Many parents enjoy the bonding experienced when they see their baby on 3D/4D imaging. The best time to obtain good 3D/4D images of your baby is between 24 and 34 weeks.
Images in earlier pregnancy (for example, on your routine 18-20 week morphology ultrasound) will appear different to the cute baby face with chubby cheeks you may have seen on TV or in a newspaper article. Babies earlier in the pregnancy do not have the same degree of subcutaneous fat. This means your baby will look less “filled out”, and more skeletal or bony, on the 3D/4D images. Many parents still enjoy seeing these early 3D/4D images, especially when their baby moves around. These images are also good to obtain on earlier ultrasounds, since many women will not need to return for a third trimester ultrasound.
Beautiful and clear 3D/4D images of your baby, especially baby’s face, are often seen on a third trimester ultrasound.
Good 3D/4D imaging requires a co-operative baby and adequate amniotic fluid in front of the structure being imaged. Some babies press against the wall of the uterus or placenta, or they may have their arms or hands lying in front of the face. This will make obtaining 3D/4D images difficult or impossible.