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Ultrasound Scanning during Pregnancy

Essentials

  • Training under a specialist is essential.
  • Do not hesitate to consult a specialist.
  • The main indications: detecting an abnormal early pregnancy, screening for foetal chromosomal and structural aberrations, monitoring foetal growth and wellbeing.
  • Foetal screening requires special skills and is often concentrated into separate foetal screening units.

Features to be observed Biophysical Profile for Fetal Assessment in High Risk Pregnancies

  • Foetal vital signs
  • Expected date of confinement (EDC, also known as expected/estimated due date, EDD)
  • Number of foetuses
  • Position of the placenta
  • Volume of amniotic fluid
  • Foetal structures
  • Assessment of foetal growth and wellbeing
  • Foetal presentation

Guidelines

  • See Screening for foetal chromosomal abnormalities Screening for Fetal Chromosomal Abnormalities.
    • Due to country-specific variation, always check also the local legislation as well as policies and practices concerning the screening for chromosomal abnormalities.
  • Participation in foetal screening is voluntary.

Recognizing pregnancy

Amniotic sac

  • An intrauterine pregnancy can be identified in the 5th week of pregnancy (WOP) with a transvaginal scan (TVS). The amniotic sac is visualized as a round clear collection of fluid.
  • With a transabdominal scan (TAS) the amniotic sac usually becomes discernible between the 7th and 9th WOP depending on the position of the uterus and the thickness of the mother's abdominal wall.
  • In practice, visualization of an intrauterine amniotic sac and a separate round yolk 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 embryo can be detected in an ultrasound scan when it is 1-2 mm long.
  • The foetal heart beat can be detected from the 5th to 6th WOP but this can happen in 5-10% of normal pregnancies slightly later.
  • The yolk sac is often seen as a separate ring-like structure in the amniotic sac.
  • The gestational age may differ from that calculated from menstruation, which should be taken into account when interpreting ultrasound scans taken in early pregnancy.
  • Multifoetal pregnancies can be detected based on the number of foetuses already in early weeks of pregnancy.

Corpus luteum cyst

  • In WOP 7-11 a separate unilocular cyst with clear content (corpus luteum cyst) can often be seen beside the uterus. Progesterone secreted by such a cyst is important for normal early pregnancy.
  • The cyst usually vanishes spontaneously towards the end of the first trimester of pregnancy.

Estimation of the expected date of confinement

  • The expected date of confinement (EDC, also known as expected/estimated due date, EDD) is usually defined on the basis of the general early pregnancy ultrasound scan.
  • If the timing of the last menstrual period is unclear, the gestational age can easily be checked by an ultrasound scan in primary health care in order to be able to schedule foetal screening.
  • The crown-rump length (CRL) is the most accurate measure for defining the EDC until 13+6 WOP (picture ).
    • The CRL should be defined by taking the mean of 3 consecutive measurements.
  • If the gestational age determined by ultrasound differs from that determined from menstruation by 5 or more days, the EDC should be corrected.
  • If the CRL is > 84 mm (14+0 WOP), a combination of several measurements should be used to assess the gestational age.
    • Biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL)
    • If the measurements differ by 7 days or more from the gestational age calculated from the last menstruation, the EDC should be changed according to the ultrasound scan.
  • The later the WOP when defining the EDC, the more uncertainty this involves.
    • The gestational age defined after 22+0 WOP is considered rather uncertain due to normal variation in foetal growth.
  • Foetal measures obtained can be compared to the gestational age in a separate table. Such tables are programmed into many ultrasound devices which then give both the gestational age and the EDC automatically.
  • In assisted pregnancies, the gestational age defined in connection with the treatment should be used.

Twin pregnancy

  • A twin pregnancy may be dizygotic (two fertilized eggs) or monozygotic (one fertilized egg).
  • Chorionicity refers to the type of placenta in a twin pregnancy.
    • In a monochorionic pregnancy, the foetuses share the placenta. The twins often have separate amnions. In such cases, the foetuses are monozygotic.
    • In a dichorionic pregnancy, the foetuses have separate placentas and separate chorions and amnions. In such cases, the foetuses may be either dizygotic or monozygotic.
    • The type of placenta is significant for assessing risks associated with the pregnancy and monitoring of pregnancy. Monochorionic pregnancies are associated with more pregnancy risks, and monitoring is frequent.
  • Chorionicity should be defined in the general early pregnancy ultrasound scan, where the diagnostic accuracy is high.
    • In dichorionic pregnancies, the lambda sign can be seen.
    • In monochorionic pregnancies, the T sign can be seen.

Nuchal translucency and combined screening in early pregnancy

  • Nuchal translucency (NT) is best visible in weeks 11+0 - 13+6 when the crown-rump length of the foetus is 45-84 mm. This is the time when the examination is best at distinguishing foetuses with trisomy from normal foetuses.
    • In combined screening, the measurement of nuchal translucency is combined with hCG and PAPP-A assays of maternal serum in weeks 9+0 - 11+6, and the age of the mother is also taken into account.
    • In this manner it is possible to detect up to 85% 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 most perpendicular lateral projection and highest magnification possible should be used (picture ).
  • A strongly deflected foetal head can give a false positive finding. A loose amniotic membrane at the dorsal side of the foetus at this stage of pregnancy can also be a source of misinterpretation. The foetal 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 for trisomy 21, 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% for miscarriage.
  • The most common trisomies can also be screened for using a maternal blood sample without an increased risk of miscarriage. The non-invasive perinatal test (NIPT) screening is based on the small amounts of cell-free foetal DNA in the mother's blood circulation. A positive NIPT result should generally be confirmed by invasive investigation.
  • Visible foetal structures should also be examined during the general early pregnancy ultrasound scan. As many as about 46% of significant structural anomalies can be detected at this stage.
  • Structural anomalies detected in the general early pregnancy ultrasound scan include anencephaly, holoprosencephaly (incomplete cleavage of the prosencephalon), abdominal wall defects (omphalocoele and gastroschisis) and abnormally large bladder.

Foetal structures (morphology)

  • The foetal structures are systematically examined on the mid-pregnancy ultrasound scan.

1. The central nervous system and spine

  • Of CNS structures, the shape of the skull, shape and width of the cerebellum, width of the posterior fossa, width of the cerebral ventricles, midline echo in the brain, and the anterior part of the brain should be checked.
    • In early pregnancy, the foetal skull is seen as an ellipsoid structure and normally with a symmetric midline echo. The lateral ventricles are visible on either side of it. BPD is measured in this plane. If an appropriate BPD cannot be measured, anencephaly should be suspected.
  • The form and closing of the spine should be assessed.
    • The spinal canal forms a zip-like structure. A clear defect in it suggests spina bifida or meningocoele.
    • Meningocoele is also strongly suggested if the cerebellum is visualized as banana-shaped (”banana-sign”), often accompanied by flattening of the frontal bones towards the midline of the skull (”lemon-sign”).
  • The neck region is examined for possible cysts and nuchal translucency (NT).
  • The most common anomalies include large cerebral ventricles, missing corpus callosum, cerebral hernia and meningocoele.
  • A cystic change in the lower spine may signify a sacral teratoma.
  • Of severe CNS anomalies, anencephaly and incomplete cleavage of the prosencephalon (holoprosencephaly) are most often detected in the general early pregnancy ultrasound scan.

2. The thorax and heart

  • In the transverse plane of the thorax the normal heart gives a four chambered view. The synchronized function of the heart and heart valves can be noted. The heart is located near the midline, one third on the right of the vertebro-sternal axis and two thirds on the left. It takes up about one third of the cross-sectional area of the thoracic cavity.
  • The ventricles and the atria are of equal size on both sides. The interventricular septum is intact and its line (the axis of the heart) is at an angle of about 45º to the vertebro-sternal axis. There is a physiological opening in the interatrial septum, the foramen ovale, with a flap opening into the left atrium.
  • In addition to the four-chamber view, the origins and normal crossing of the great vessels, i.e. the aorta and the pulmonary artery, should be assessed. In the 3-vessel plane, cross-sectional views of the pulmonary artery, the aorta and the superior vena cava can be seen. A great deal of structural abnormalities of the heart can be excluded if the four-chamber view and the crossing of the vessels are normal.
  • The aortic and ductal arches and the connection of the superior and inferior vena cava to the right atrium can be seen in a longitudinal view.
  • The pulmonary tissue is homogenously echogenic. If this is not the case, a diaphragmatic hernia (often associated with an atypical position of the heart) can be suspected. Cystic changes may suggest congenital pulmonary airway malformation (CPAM).

3. Closure of the abdominal wall

  • Closure of the abdominal wall should be checked particularly at the insertion of the umbilical cord for the exclusion of omphalocoele or gastroschisis.

4. Organs in the abdominal cavity

  • The stomach usually forms a bean-shaped structure on the left, beneath the diaphragm. An extra accumulation of fluid beside the stomach (”double-bubble”) suggests duodenal stenosis. If no stomach can be seen, oesophageal atresia should be suspected.
  • The liver and kidneys, and the width of the renal pelvis should be checked. Hydronephrosis is the most common urinary tract anomaly. It is most commonly due to obstruction of the ureteropelvic junction and vesicoureteral reflux. Mild hydronephrosis may resolve after delivery without any residual anatomic anomaly.
  • The bladder can be seen in the pelvis, and the umbilical arteries run along either side of the bladder. The ureters cannot normally be seen.
  • Severe anomalies in the urinary tract - such as the absence of kidneys or a urethral valve - are associated with no or low amniotic fluid.
  • Fluid accumulation in the abdomen, other than the bladder and stomach, should prompt further investigation. For example, a fluid accumulation visualized beside the urinary bladder in a female foetus may be an ovarian cyst, which is usually harmless.

5. The extremities

  • The number of long bones (12), the hands and the position of the ankles should be noted, as well as foetal body movement.
  • Club foot is one of the most common anomalies in the extremities. Slender long bones may signify a bone disease.

6. Face

  • In the facial area, cleft lip should be excluded to the extent possible. The profile of the foetal face should also be checked.

7. Gender

  • Usually there is no clinical indication for identifying foetal gender; 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 may cause false interpretations of gender.

8. Foetal growth

  • Foetal growth can be estimated based on biparietal diameter (BPD) and head circumference, abdominal circumference and femur length, and the foetal weight estimated based on these.

The placenta, umbilical cord and amniotic fluid Amniotic Fluid Index Versus Single Deepest Vertical Pocket as a Screening Test for Preventing Adverse Pregnancy Outcome

  • The position of the placenta needs to be determined and the position of the lower edge of the placenta in relation to the internal os of uterus estimated.
    • A placenta attached low down in the uterus may cover the cervical opening completely (complete placenta previa) or partially (partial placenta previa) or be situated marginally by the cervical opening.
    • The edge of the placenta situated < 2 cm from the cervical opening indicates a low-lying placenta.
  • Complete placenta previa means a high-risk pregnancy that should be followed in specialized care.
    • The position of a partial placenta previa or a low-lying placenta is often checked only in late pregnancy to assess the mode of delivery. At that point, a significant share of placentas have moved to a region that is safe for vaginal delivery, due to factors such as stretching of the uterine isthmus.
  • In early pregnancy, the composition of amniotic fluid resembles that of foetal plasma, which is diffused through the foetal skin to produce the amniotic fluid surrounding the foetus. From about 14 WOP, amniotic fluid consists increasingly of foetal urine and foetal pulmonary secretions. Some amniotic fluid is lost as it is swallowed by the foetus.
  • The amniotic fluid protects the foetus from compression of the umbilical cord, and facilitates foetal movement and normal development of the limbs and joints. It is also essential for normal foetal pulmonary maturation.
  • The amount of amniotic fluid is estimated by, for example, measuring the diameter of the largest single fluid pocket Amniotic Fluid Index Versus Single Deepest Vertical Pocket as a Screening Test for Preventing Adverse Pregnancy Outcome. The greatest diameter to be considered normal in mid-pregnancy is 2-8 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. In colour Doppler examination, 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 chromosomal abnormalities, other structural anomalies or foetal growth retardation.

The cervical canal

  • To increase the accuracy of measurement, the length of the cervical canal should preferably be measured by transvaginal ultrasound.
  • In mid-pregnancy, the average length of the cervical canal is 36 mm, but there is individual variation. The risk of preterm birth increases if the cervical canal is < 25 mm long, and cervical canal length < 15 mm is clearly abnormal.
  • Extensive dilation of the internal orifice of the cervical canal may also suggest impending preterm birth.

Ultrasound markers for trisomy in mid-pregnancy

  • In addition to structural anomalies, an ultrasound examination may show what are called ‘soft markers', or minor ultrasound findings. If the pregnant woman has participated in combined screening or had an NIPT with normal results, a single soft marker finding will not increase the risk of trisomy significantly.
  • The most common soft markers include
    • Plexus choroideus cysts
    • Echo-dense dots in the papillary muscles of the foetal heart
    • Echo-dense intestine
    • Mild hydronephrosis
    • Short femur

Foetal growth

  • Routine late pregnancy ultrasound in unselected or low-risk populations is not necessary Routine Ultrasound in Late Pregnancy (after 24 Weeks Gestation).
  • Rapidly growing BPD may suggest hydrocephalus and slowly growing BPD 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.

Estimation of foetal weight

  • Abdominal circumference measured as symmetrically as possible in the plane of the umbilical vein (sinus umbilicalis) and stomach is the most important parameter. 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, BPD or head circumference and femur length.
  • In a large foetus, a small BPD and large abdominal circumference indicate an increased risk of shoulder dystocia at birth.
  • Before week 30, a weight estimate has not much significance in a normal pregnancy; half of the weight of a full-term foetus is formed during the last 8-10 weeks of pregnancy.

Foetal Doppler ultrasound Doppler Ultrasound for Foetal Assessment in High Risk Pregnancies

  • Doppler ultrasound is used to monitor foetal wellbeing in the case of intrauterine growth retardation. It is most often done on the umbilical artery, middle cerebral artery and the ductus venosus. The aim is to assess an appropriate time of delivery particularly in preterm weeks of pregnancy.
  • The flow rate in the middle cerebral artery is measured if foetal anaemia is suspected.

Presentation

  • After the 35th WOP anything other than a cephalic presentation is an indication for referring the patient to specialized care.

    References

    • Jauniaux E, Alfirevic Z, Bhide AG, et al. Placenta Praevia and Placenta Accreta: Diagnosis and Management: Green-top Guideline No. 27a. BJOG 2019;126(1):e1-e48 [PubMed]
    • Methods for Estimating the Due Date. American College of Obstetricians and Gynecologists (ACOG) Committee on Obstetric Practice,American Institute of Ultrasound in Medicine,Society for Maternal-Fetal Medicine. Committee Opinion 700, 2017, reaffirmed 2022 http://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
    • Karim JN, Roberts NW, Salomon LJ, et al. Systematic review of first-trimester ultrasound screening for detection of fetal structural anomalies and factors that affect screening performance. Ultrasound Obstet Gynecol 2017;50(4):429-441 [PubMed]
    • Kagan KO, Sonek J. How to measure cervical length. Ultrasound Obstet Gynecol 2015;45(3):358-62 [PubMed]
    • Salomon LJ, Alfirevic Z, Bilardo CM, et al. ISUOG practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 2013;41(1):102-13 [PubMed]