LEVEL II ULTRASOUND
What are the indications or reasons for this scan?
- A detailed examination of the anatomy of all the fetal systems
- Detection of anatomical markers that increase the risk of chromosomal abnormalities.
- Assessment of normal intrauterine growth.
- Determining the position of the placenta.
- Estimating the volume of amniotic fluid.
- The first detection of twin–twin transfusion syndrome in monochorionic twin pregnancies.
- Detailed examination of the heart in fetuses whose nuchal translucency was found to be increased in the first trimester.
- Assessment of the blood flow in the uterine vessels and classification of the pregnancy as high risk (where the flow is abnormal) for intrauterine growth restriction or preeclampsia, which calls for closer monitoring of the pregnancy.
- Every pregnant woman has a transvaginal scan to measure the cervix. This measurement is used to predict the risk of preterm birth.
The procedure — parents are advised to have the scan at 22 weeks, since by then the fetus is the right size for the operator to make out its anatomy more easily.
The scan is done transabdominally (through the mother's abdomen). The level II examination is carried out using specific ultrasound views, as laid down in the protocols of the Fetal Medicine Foundation (FMF), which have been shown to have the highest sensitivity for detecting possible fetal abnormalities.
The fetal anatomy is examined system by system, covering all the vital organs. The following are therefore examined in separate ultrasound views:
Head: we focus on the shape of the skull and on normal bone density. Brain: we examine the cerebral hemispheres, the ventricles and the cerebellum. Face: we look at the lenses of the eyes, the nose and nostrils, the palate, the lower jaw, the lips, the ears and the facial profile. Neck: we measure the thickness of the nuchal fold. Spine: we assess the shape, the curvature and the bone density of the spine and its vertebrae. Chest and lungs: we check the shape of the chest and the integrity of the diaphragm. Heart: we see the four chambers of the heart, the three vessels of the chest, the aortic arch and the origins of the aorta and the pulmonary artery. Abdominal wall: we confirm that the abdominal wall is intact. Gastrointestinal system: we grade the echogenicity of the bowel and examine the abdominal organs one by one.
Urinary system and kidneys: we confirm that both kidneys are present in their normal anatomical position and examine the filling of the bladder in detail. Arms and legs: we measure the length of the humerus, radius, ulna, femur, tibia and fibula, the fingers and the feet, and assess their shape and bone density.
Genitalia and sex: we confirm the sex of the baby.
During the detailed level II anatomy scan the minor abnormalities, or markers for chromosomal abnormalities, are also assessed. The main ones are prenasal oedema, echogenic bowel, absence or hypoplasia of the nasal bone, an increased nuchal fold, hydrocephalus and an aberrant right subclavian artery.
The presence of one or more markers is not usually a problem in itself, but it does increase the probability that the fetus has a chromosomal abnormality. It is worth stressing that the detailed level II anatomy scan has a low sensitivity for detecting chromosomal abnormalities: 40% of babies with Down syndrome, for example, appear normal on this examination.
Chromosomal abnormalities are screened for at the 11–13 week scan (nuchal translucency). The various level II ultrasound markers are considered 'minor' and, if present, are used only to revise the probability of a chromosomal abnormality as it was calculated in the first trimester.
In women at high risk of preterm birth (multiple pregnancies, a history of preterm birth, abnormalities of the uterus or cervix, previous surgery) we perform a transvaginal scan to measure the length of the cervix.
The last part of the examination concerns the blood flow in the mother's uterine arteries and shows us how well the placenta is being supplied with blood. When the flow in these vessels shows high resistance — in other words, it is impeded — the chance increases that the mother will develop high blood pressure later in pregnancy, or that the fetus will not grow adequately. Factors from the mother's previous history are taken into account as well. Although in most of these cases the pregnancy ultimately progresses without problems, as a precaution we recommend regular monitoring of the mother's blood pressure and of the baby's growth.
Where the fetus is in a suitable position, images are also taken using three-dimensional (3D) ultrasound. The examination takes about 60 minutes. How effective is the level II scan?
You should know that this scan detects many congenital abnormalities of the fetus, but not all of them. In practice this means that a normal level II scan considerably reduces the risk that the fetus has a serious abnormality, without eliminating it. Even in the most experienced hands, abnormalities can be missed — either because they were not visible at the time the level II scan was performed, or because their nature makes them extremely hard to diagnose, or because for practical reasons (an obese mother, a poor fetal position and so on) the diagnosis was not possible.
Below is an indicative table of congenital abnormalities and the probability that they will be diagnosed on the level II scan.