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Ultrasound Scanning during Pregnancy
Essentials
- Training under a specialist is essential (exceptions: foetal pulse on weeks 7-9 of pregnancy and presentation in late pregnancy).
- Do not hesitate to consult a specialist.
Features to be observed
- Expected date of confinement, EDC (the most important and easiest to carry out)
- Number of foetuses
- Position of the placenta
- Foetal structures, morphology
- Presentation, when needed (easy to carry out)
- Growth if deviation is suspected
Guidelines
- The time of the first routine scan is agreed upon locally and depends on the mode of trisomy screening.
- The current national recommendation in Finland is that all pregnant women are offered an ultrasound scan in the early pregnancy (week 11 to 13) to establish the duration of pregnancy Ultrasound for Fetal Assessment in Early Pregnancy and the number of foetuses Ultrasound for Fetal Assessment in Early Pregnancy and to reveal major foetal abnormalities. If the mother so wishes, also measurement of nuchal translucency and determination of blood hCG and PAPP-A concentrations are included to assess the risk of trisomy. In addition, a mid-pregnancy ultrasound scan is offered for systematic investigation of foetal morphology around the 20th to 22nd week of pregnancy.
Recognizing pregnancy
Amniotic sac
- An intrauterine amniotic sac can be identified on the 5th week of pregnancy (WOP) with a transvaginal scan (TVS). The sac is visualized as a round clear area in the uterine cavity .
- With a transabdominal scan (TAS) the amniotic sac usually becomes discernible between the 7th and 9th WOP depending on the thickness of mother's abdominal wall and the position of the uterus.
- In practice, visualization of an intrauterine amniotic sac rules out the possibility of an extrauterine pregnancy.
- See also Gynaecological Ultrasound Examination
The embryo
- First seen as a small dense echo within the amniotic sac.
- The foetal heart beat can be detected as a barely visible flutter already when the foetus in only a few millimetres long.
- The yolk sac is seen as a separate ring-like structure in the amniotic sac .
Multifoetal pregnancies
- A twin pregnancy can be determined in early pregnancy. One embryo can, however, be aborted, which manifests as bleeding in early pregnancy.
- The twins are usually dizygotic if the placental tissue penetrates between the layers of the placental insertion of the separating membrane ("twin peak" or lambda sign). If the thickness of the separating membrane is less than 2 mm, the twins are likely to be monozygotic. It may sometimes be possible to count the number of layers of the separating membrane (two in monozygotic and four in dizygotic twins).
Corpus luteum cyst
- On WOP 7-11 a separate unilocular clear, thin, walled cyst measuring 2-4 cm is often seen beside the uterus. This vanishes later on and needs no intervention.
Estimation of the expected date of confinement
- Ultrasound scan before 20 WOP is the most reliable method for determining EDC.
- Accuracy is best on 10-12 WOP ±3-4 days, at other times ±7 days. If the time determined by ultrasound differs from that determined from menstruation by 5 or more days, EDC should be corrected.
- The crown-rump-length (CRL) is used to estimate gestational age before 13 WOP (picture ).
- After 11 WOP biparietal diameter (BPD; picture ) or the length of the diaphysis of the femur (femur length) or both are used.
- The gestational age corresponding to the obtained measures is given in tables that are programmed in to many ultrasound devices. Such devices give both the gestational age and EDC automatically.
Nuchal translucency and combined screening in early pregnancy
- Nuchal translucency (NT) is best visible in the weeks 11+2 - 13+6 when the crown-rump length of the foetus is 45-84 mm, which is the time when the examination is at its best in distinguishing foetuses with trisomy from normal foetuses. In the combined screening, the measurement of nuchal translucency is combined with hCG and PAPP-A assays of maternal serum in the weeks 9-11, and the age of the mother is also taken into account. In this manner it is possible to detect up to 80% of all trisomies, and the proportion of false positive results is less than 3% Screening for Fetal Chromosomal Abnormalities.
- NT is always measured from the inner edge of the skin to the outer edge of the underlying tissue, i.e. the shortest possible distance. The best possible side profile and image magnification should be used (picture ).
- Strongly deflected fetal head can give a false positive finding. A loose amniotic membrane at the dorsal side of the foetus can also be a source for misinterpretation. Fetal nasal bone may also be visualized during the same scan. If this can be seen, the risk for a trisomy 21 is very low.
- If the risk calculation shows a ratio of 1:250 or greater it is considered positive, and the chromosomes of the foetus can be examined by performing chorionic villus sampling or amniocentesis, if the mother so wishes. These are invasive investigations with a risk of 0.5-1% for miscarriage. Nowadays, the most common trisomies can be screened also using a maternal blood sample without increased the risk of miscarriage. The non-invasive perinatal test (NIPT) screening is based on the small amounts of fetal cell-free DNA in the mother's blood circulation. A positive NIPT result should always be confirmed by amniocentesis.
Foetal structures (morphology)
- The structures are systematically examined on the mid-pregnancy ultrasound scan.
1. The head and spinal canal
- In the transverse plane the foetal skull is seen as an ellipsoid structure with a symmetric mid-echo. The lateral ventricles are visible on either side of it. Their width is not more than half of the inner width of the hemicranium. BPD is measured in this plane. If a good BPD cannot be achieved, anencephaly should be suspected.
- Normally, symmetrical dense echoes, choroid plexuses, are seen on both sides of the mid-echo. In some cases, a choroid plexus cyst may be identified; these are usually harmless. If the echoes are asymmetrical or non-homogeneous and the duration of gestation counted from menstruation and femur length differs clearly from that estimated from BPD, further investigations are warranted.
- In the area of the posterior cranial fossa, the peanut-shaped cerebellum is visualized. The transverse diameter of the cerebellum in millimetres usually corresponds to the duration of the pregnancy in weeks. The hypoechoic area between the cerebellum and the occipital bone, the cisterna magna, is visualized as well (maximum width 9 mm). In the inspection of the posterior cranial fossa, the posterior horn of the lateral ventricle is often also visualized; its maximum width is also 9 mm. A neural tube defect is strongly suggested if the cerebellum is visualized as banana-shaped (”banana-sign”), often accompanied by the flattening of the frontal bones towards the midline of the skull (”lemon-sign”).
- In the sagittal plane, a possible encephalocoele can visualized. The sagittal profile of the foetal face is also observed: a flat profile may suggest trisomy.
- The coronal view of the foetal face should normally be symmetrical. The nasal region and the upper lip are examined to exclude cleft lip.
- The spinal canal forms a zip-like structure. A clear defect in it suggests spina bifida or meningocoele or both (see above the banana and lemon signs).
- The neck region is examined for possible cysts and nuchal translucency (NT).
2. The outline of the foetal body
- Any abnormality on the dorsal side is usually seen upon inspection of the spinal canal.
- In the ventral outline, attention should be paid to the insertion of the umbilical cord for possible omphalocele or gastrochisis in the abdominal wall.
- A greater magnification is used to look for sacral teratoma.
- Foetal body movement should be noted.
3. The thorax and heart
- In the transverse plane of the thorax the normal heart gives a four chambered view. The synchronized function of the atria, ventricles and valves should be noted. The heart is located near the midline, one third on the right side and two thirds on the left side of the vertebro-sternal axis. It takes up about one third of the cross-sectional area of the thoracic cavity.
- The ventricles and the atriae are of equal size on both sides. The interventricular septum is visualized intact and its line (the axis of the heart) is at an angle of about 45º with the vertebro-sternal axis. There is a physiological defect, foramen ovale, in the interatrial septum with a membrane bulging slightly towards the left atrium.
- The origins and the normal crossing of the great vessels, i.e. aorta and pulmonary artery, are seen slightly cranially from the four-chamber view. A bit more cranially the 3-vessel view can be seen, i.e. the cross-sectional views of the ascending and descending aorta and the superior vena cava. A great deal of structural abnormalities of the heart may be excluded if the four-chamber view and the crossing of the vessels are normal.
- Small echo-dense spots (golf balls) in the area of the papillary muscles suggest a slightly increased risk of trisomy.
- The pulmonary tissue is homogenous in echodensity. If this is not the case, a diaphragmatic hernia (often associated with atypical position of the heart) or cystic adenomatoid malformation of the lung should be suspected.
- In sagittal inspection of the thoracic cavity, the aortic and ductal arches should be visualized; when intact, the former has the shape of a walking stick, the latter the form of a hockey stick.
4. The abdominal cavity
- The ventricle forms an echo-free, bean-shaped structure on the left, beneath the diaphragm and this finding also indicates a patent oesophagus. An extra accumulation of fluid beside the ventricle (”double-bubble”) suggests duodenal stenosis.
- Liver and kidneys are not easy to identify before 20 WOP. A fluid-filled bladder at the caudal end of the cavity indicates normal function of at least one kidney and ureter. If the bladder cannot be visualized but the amount of amniotic fluid is normal, control the finding. The foetus empties its bladder every 15 to 20 minutes.
- Fluid accumulation in the abdomen, other than the ventricle and bladder, indicate further investigation. E.g. a fluid accumulation visualized beside the urinary bladder in a female foetus may be an ovarian cyst, which is usually harmless.
- Echo-dense (density equal to that of bones) intestines and/or mild pyelectasia suggest increased risk of trisomy.
5. The extremities
- In addition to biparietal length, the length of the femur is an important measure when determining gestational age on weeks 15-19 of pregnancy. A considerable discrepancy (more than 2 weeks of pregnancy) between these measures warrants further investigations.
- The outline of the limbs, hands, ankles (club foot) and feet, the position of the wrists and ankles should be noted.
- Foetal body movement should be noted.
- A low-lying placenta (picture ) is a common finding in early and mid-pregnancy. The position of the placenta needs to be determined on weeks 28-30. However, as the isthmic portion of the uterus usually grows more than the other parts, the placenta seems to "migrate" upwards.
- The identification of the lower end of the placenta is easier with full maternal bladder.
- A back-wall placenta is seen better with transvaginal ultrasound.
- In early pregnancy the amniotic fluid is formed mainly by the amniotic membranes.
- In mid- and late pregnancy the fluid results from foetal metabolism, predominantly urine. Severe oligohydramnios in mid-pregnancy, irrespective of the aetiology, is associated with poor prognosis due to the fact that a sufficient amount of amniotic fluid is essential for foetal pulmonary maturation.
- The amount of the amniotic fluid is estimated by measuring the diameter of the single deepest fluid pocket Amniotic Fluid Index Versus Single Deepest Vertical Pocket as a Screening Test for Preventing Adverse Pregnancy Outcome. The diameter to be considered normal in the the second and third trimesters measures 3-8 cm.
- An abnormal amount of amniotic fluid is an indication for further investigations.
- In a cross-section of a normal umbilical cord, three vessels can be seen. If a colour Doppler device is available, the two umbilical arteries can also be visualized by inspecting the bladder region of the foetus: the arteries run on either side of the bladder. A single umbilical artery can be associated with other vascular (or urinary) anomalies and warrants careful examination of foetal structures.
7. The cervix
- In early and midpregnancy, the cervix is quite easy to see if the maternal bladder is full. If the length of the cervical canal is less than 30 mm, or the proximal part is dilated, cervical incompetence should be suspected. Funneling of the internal orifice of the cervix is better visualized if pressure is applied on the bottom of the uterus during the examination.
8. Gender
- There are very few clinical indications for identifying foetal sex; the genitals are, though, a part of the foetal morphology.
- Labia suggest a female foetus and echo-dense testes that have descended to the scrotum and penis suggest a male. Umbilical cord between the legs easily causes false interpretations of gender.
Ultrasound markers for trisomy in mid-pregnancy
- As single findings, the following markers increase the risk for trisomy only slightly. However, if two or more markers are present in one foetus, foetal karyotyping should be considered.
- Plexus choroideus cysts
- Flat side profile of the face
- Echo-dense dots in the papillary muscles of the foetal heart ("golf balls")
- Echo-dense intestine
- Mild hydronephrosis (diameter of the renal pelvis in anteroposterior direction over 6 mm)
- Growth retardation
- Short femur
- Umbilical cord cysts.
Foetal growth
- On the latter half of pregnancy the growth and development are followed up in addition to foetal structures.
- Routine ultrasound screening in late pregnancy is not necessary in low-risk pregnancies or without a clear target Routine Ultrasound in Late Pregnancy (after 24 Weeks Gestation).
- Rapidly growing BPD may suggest hydrocephalus and slowly growing microcephaly or some other CNS disease.
- Retarded growth of the foetal abdominal circumference with normally growing BPD is often a sign of impaired function of the placenta. Excessive growth of the body may suggest foetal hydrops.
- Retarded growth of the limbs warrants further investigations.
Estimation of weight
- Measurement of abdominal circumference is the most important parameter for weight estimation. This should be measured as symmetrically as possible from the plane of the foetal liver, sinum umbilicalis and ventricle. Several measurements should be made and the average should be used in the final estimation.
- Many programs give an estimate automatically on the basis of abdominal circumference and BPD.
- In the beginning of the third trimester, BPD correlates well with foetal weight, however, towards the end and, especially if foetal gigantism is suspected, femur length is a more accurate measure.
- In a large-sized foetus, small BPD and great abdominal circumference indicate an increased risk of getting stuck at the shoulders at birth.
- Before week 30, a weight estimate has rather little significance; half of the weight of a full-term foetus is formed during the last 8-10 weeks of pregnancy.
Presentation
- After the 35th WOP anything other than a cephalic presentation is an indication for an obstetric consultation.
Post-term pregnancy
- Decreasing amniotic fluid volume is considered to correlate better with deteriorating placental function than structural changes (calcification and lobularity) in the placenta.
Doppler ultrasound of the umbilical artery