What Are Obstetric Ultrasound Scans

8/8/2019 What Are Obstetric Ultrasound Scans

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What are Obstetric Ultrasound Scans?

Obstetric Ultrasound is the use of ultrasound scans inpregnancy. Since its introduction in the late 1950sultrasonography has become a very useful diagnostic tool inObstetrics.

Currently used equipments are known as real-timescanners, with which a continous picture of the movingfetus can be depicted on a monitor screen. Very highfrequency sound waves of between 3.5 to 7.0 megahertz(i.e. 3.5 to 7 million cycles per second) are generally used

for this purpose.

They are emitted from a transducer which is placed in contact with the maternal abdomen,and is moved to "look at" (likened to a light shined from a torch) any particular content ofthe uterus. Repetitive arrays of ultrasound beams scan the fetus in thin slices and are

reflected back onto the same transducer.

The information obtained from different reflections arerecomposed back into a picture on the monitor screen (asonogram, or ultrasonogram). Movements such as fetal heartbeat and malformations in the feus can be assessed andmeasurements can be made accurately on the imagesdisplayed on the screen. Such measurements form thecornerstone in the assessment of gestational age, size andgrowth in the fetus.

A full bladder is often required for the procedure when

abdominal scanning is done in early pregnency. There may be some discomfort frompressure on the full bladder. The conducting gel is non-staining but may feel slightly coldand wet. There is no sensation at all from the ultrasound waves.

A short history of the development of ultrasound in pregnancy can be found in the Historypages.

Why and when is Ultrasound used in Pregnancy?

Ultrasound scan is currently considered to be a safe, non-invasive, accurate and cost-effective investigation in the fetus. It has progressively become an indispensible obstetrictool and plays an important role in the care of every pregnant woman.


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The main use of ultrasonography are in the following areas:

1. Diagnosis and confirmation of early pregnancy.

The gestational sac can be visualized as early as four and ahalf weeks of gestation and the yolk sac at about five weeks.The embryo can be observed and measured by about five anda half weeks. Ultrasound can also very importantly confirm thesite of the pregnancy is within the cavity of the uterus.

2. Vaginal bleeding in early pregnancy.

The viability of the fetus can be documented in the presence ofvaginal bleeding in earlypregnancy. A visible heartbeat could be seen and detectable by pulsed doppler ultrasoundby about 6 weeks and is usually clearly depictable by 7 weeks. If this is observed, theprobability of a continued pregnancy is better than 95 percent. Missed abortions and

blighted ovum will usually give typical pictures of a deformed gestational sac and absence offetal poles or heart beat.

Fetal heart rate tends to vary with gestational age in the very early parts of pregnancy.Normal heart rate at 6 weeks is around 90-110 beats per minute (bpm) and at 9 weeks is140-170 bpm. At 5-8 weeks a bradycardia (less than 90 bpm) is associated with a high riskof miscarriage.

Many women do not ovulate at around day 14, so findingsafter a single scan should always be interpreted with caution.The diagnosis of missed abortion is usually made by serialultrasound scans demonstrating lack of gestational

development. For example, if ultrasound scan demonstrates a7mm embryo but cannot demonstrable a clearcut heartbeat, amissed abortion may be diagnosed. In such cases, it isreasonable to repeat the ultrasound scan in 7-10 days toavoid any error.

The timing of a positive pregnancy test may also be helpful inthis regard to assess the possible dates of conception. A positive pregnancy test 3 weekspreviously for example, would indicate a gestational age of at least 7 weeks. Suchinformation would be useful against the interpretation of the scans. Please read the FAQsfor more comments.

In the presence of first trimester bleeding, ultrasonography is also indispensible in the earlydiagnosis ofectopic pregnancies and molar pregnancies.

3. Determination of gestational age and assessment of fetal size.

Fetal body measurements reflect the gestational age of the fetus. This is particularly true inearly gestation. In patients with uncertain last menstrual periods, such measurements mustbe made as early as possible in pregnancy to arrive at a correct dating for the patient. See


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FAQ. In the latter part of pregnancy measuring body parameters will allow assessment ofthe size and growth of the fetus and will greatly assist in the diagnosis and management ofintrauterine growth retardation (IUGR).

The following measurements are usually made:

a) The Crown-rump length (CRL)

This measurement can be made between 7 to 13weeks and gives very accurate estimation of thegestational age. Dating with the CRL can bewithin 3-4 days of the last menstrual period.(Table) An important point to note is that whenthe due date has been set by an accuratelymeasured CRL, it should not be changed by asubsequent scan. For example, if another scandone 6 or 8 weeks later says that one shouldhave a new due date which is further away, one

should not normally change the date but should rather interpret the finding asthat the baby is not growing at the expected rate.

b) The Biparietal diameter (BPD)

The diameter between the 2 sides of the head. This is measured after 13weeks. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term.Different babies of the same weight can have different head size, thereforedating in the later part of pregnancy is generally considered unreliable. (Chartand further comments) Dating using the BPD should be done as early as isfeasible.

c)The Femur length (FL)

Measures the longest bone in the body and reflects the longitudinal growth ofthe fetus. Its usefulness is similar to the BPD. It increases from about 1.5 cmat 14 weeks to about 7.8 cm at term. (Chart and further comments) Similarto the BPD, dating using the FL should be done as early as is feasible.

d) The Abdominal circumference (AC)

The single most important measurement to make in late pregnancy. Itreflects more of fetal size and weight rather than age. Serial measurementsare useful in monitoring growth of the fetus. (Chart and further comments)AC measurements should not be used for dating a fetus.

Other important measurements are discussed here.

The weight of the fetus at any gestation can also beestimated with great accuracy using polynomial equationscontaining the BPD, FL, and AC. computer softwares andlookup charts are readily available. For example, a BPD of 9.0


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cm and an AC of 30.0 cm will give a weight estimate of 2.85 kg. (comments)

4. Diagnosis of fetal malformation.

Many structural abnormalities in the fetus can be reliably

diagnosed by an ultrasound scan, and these can usually bemade before 20 weeks. Common examples includehydrocephalus, anencephaly, myelomeningocoele,achondroplasia and other dwarfism, spina bifida, exomphalos,Gastroschisis, duodenal atresia and fetal hydrops. With morerecent equipment, conditions such as cleft lips/ palate andcongenital cardiac abnormalities are more readily diagnosedand at an earlier gestational age. (Also see the FAQ andAnomalies pages).

First trimester ultrasonic 'soft' markers for chromosomalabnormalities such as the absence offetal nasal bone, an increased fetal nuchal

translucency (the area at the back of the neck) are now in common use to enable detectionofDown syndrome fetuses.

Read also: Soft Markers - A Guide for Professionals and Ultrasonographic "soft markers" offetal chromosomal defects.

Ultrasound can also assist in other diagnostic procedures inprenatal diagnosis such as amniocentesis, chorionic villussampling, cordocentesis (percutaneous umbilical blood sampling)and in fetal therapy.

5. Placental localization.

Ultrasonography has become indispensible in thelocalization of the site of the placenta and determining itslower edges, thus making a diagnosis or an exclusion ofplacenta previa. Other placental abnormalities inconditions such as diabetes, fetal hydrops, Rhisoimmunization and severe intrauterine growthretardation can also be assessed.

6. Multiple pregnancies.

In this situation, ultrasonography is invaluable indetermining the number of fetuses, the chorionicity, fetal presentations, evidence of growthretardation and fetal anomaly, the presence of placenta previa, and any suggestion oftwin-to-twin transfusion.

7. Hydramnios and Oligohydramnios.


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Excessive or decreased amount of liquor (amniotic fluid) can be clearly depicted byultrasound. Both of these conditions can have adverse effects on the fetus. In both thesesituations, careful ultrasound examination should be made to exclude intraulterine growthretardation and congenital malformation in the fetus such as intestinal atresia, hydropsfetalis or renal dysplasia. See also FAQ and comments.

8. Other areas.

Ultrasonography is of great value in other obstetric conditions such as:

a) confirmation of intrauterine death.b) confirmation of fetal presentation in uncertain cases.c) evaluating fetal movements, tone and breathing in the Biophysical Profile.d) diagnosis of uterine and pelvic abnormalities during pregnancy e.g.fibromyomata and ovarian cyst.

Transvaginal Scans

With specially designed probes, ultrasound scanning can bedone with the probe placed in the vagina of the patient. Thismethod usually provides better images (and therefore moreinformation) in patients who are obese and/ or in the earlystages of pregnancy. The better images are the result of the

scanhead's closer proximity to the uterus and the higherfrequency used in the transducer array resulting in higherresolving power. Fetal cardiac pulsation can be clearly

observed as early as 6 weeks of gestation.

Vaginal scans are also becoming indispensible in the early diagnosis ofectopic pregnancies.An increasing number of fetal abnormalities are also being diagnosed in the first trimesterusing the vaginal scan. Transvaginal scans are also useful in the second trimester in thediagnosis of congenital anomalies. Read one of my presentations at OBGYN.net-Ultrasound.

Doppler Ultrasound

The doppler shift principle has been used for a long time infetal heart rate detectors. Further developments in dopplerultrasound technology in recent years have enabled a greatexpansion in its application in Obstetrics, particularly in thearea of assessing and monitoring the well-being of the


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fetus, its progression in the face of intrauterine growth restriction, and the diagnosis ofcardiac malformations.

Doppler ultrasound is presently most widely employed in the detection of fetal cardiacpulsations and pulsations in the various fetal blood vessels. The "Doptone" fetal pulsedetector is a commonly used handheld device to detect fetal heartbeat using the same

doppler principle.

Blood flow characteristics in the fetal blood vessels can beassessed with Doppler 'flow velocity waveforms'. Diminishedflow, particularly in the diastolic phase of a pulse cycle isassociated with compromise in the fetus. Various ratios of thesystolic to diastolic flow are used as a measure of thiscompromise. The blood vessels commonly interrogated includethe umbilical artery, the aorta, the middle cerebral arteries, theuterine arcuate arteries, and the inferior vena cava.

The use ofcolor flow mapping can clearly depict the flow of

blood in fetal blood vessels in a realtime scan, the direction ofthe flow being represented by different colors. Color doppler is

particularly indispensible in the diagnosis offetal cardiac and blood vessel defects, and inthe assessment of the hemodynamic responses to fetal hypoxia and anemia.

A more recent development is the Power Doppler (Doppler angiography). It uses amplitudeinformation from doppler signals rather than flow velocity information to visualize slow flowin smaller blood vessels. A color perfusion-like display of a particular organ such as theplacenta overlapping on the 2-D image can be very nicely depicted. Doppler examinationscan be performed abdominally and via the transvaginal route. The power emitted by adoppler device is greater than that used in a conventional 2-D scan. Its use in earlypregnancy is therefore cautioned.

Doppler facilities are generally an integral part of modern ultrasound scanners. They merelywould need to be switched on to function. One does not need to 'go' to another machine forthe doppler investigations.

3-D and 4-D Ultrasound

3-D ultrasound can furnish us with a 3 dimensional image of what we arescanning. The transducer takes a series of images, thin slices, of the subject,and the computer processes these images and presents them as a 3 dimensional

image. Using computer controls, the operator can obtain views that might not be availableusing ordinary 2-D ultrasound scan. 3-dimensional ultrasound is quickly moving out of theresearch and development stages and is now widely employed in a clinical setting. It too, isvery much in the News. Faster and more advanced commercial models are coming into themarket. The scans requires special probes and software to accumulate and render theimages, and the rendering time has been reduced from minutes to fractions of a seconds.

A good 3-D image is often very impressive to the parents. Further 2-D scans may beextracted from 3-D blocks of scanned information. Volumetric measurements are more


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accurate and both doctors and parents can better appreciate a certain abnormality or theabsence of a certain abnormality in a 3-D scan than a 2-D one and there is the possibility of

increasing psychological bonding between the parents and the baby.

An increasing volume of literature is accumulating on the usefulness of3-D scans and the diagnosis of congenital anomalies could receive

revived attention. Present evidence has already suggested that smallerdefects such as spina bifida, cleft lips/palate, and polydactyl may bemore lucidly demonstrated. Other more subtle features such as low-setears, facial dysmorphia or clubbing of feet can be better assessed,leading to more effective diagnosis of chromosomal abnormalities. Thestudy of fetal cardiac malformations is also receiving attention. Theability to obtain a good 3-D picture is nevertheless still very muchdependent on operator skill, the amount of liquor (amniotic fluid)around the fetus, its position and the degree of maternal obesity, so

that a good image is not always readily obtainable.

More recently, 4-D or dynamic 3-D scanners are in the

market and the attraction of being able to look at the face andmovements of your baby before birth was also enthusiasticallyreported in parenting and health magazines. This is thought tohave an important catalytic effect for mothers to bond to theirbabies before birth. What are known as 're-assurance scans'and the rather misnamed 'entertainment scans' have quicklybecome popular.

Most experts do not consider that 3-D and 4-D ultrasound willbe a mandatory evolution of our conventional 2-D scans, rather it is an additional piece oftool like doppler ultrasound. Most diagnosis will still be made with the 2-D scans. 3-Dultrasound appears to have great potential in research and in the study offetal embryology.Whether 3-D ultrasound will provide unique information or merely supplemental information

to the conventional 2-D scans will remain to be seen.

Move cursor over the image to see highlighted pathology


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Choroid Plexus Cysts

Common finding with modern equipment.

Is a soft marker if found in association with

2 or more other soft markers for trisomy 18

No significance in isolation and usually

disappear in the 3rd trimester

Hydrocephalus

Holoprosencephaly

Alobar holoprosencephaly is the most

severe form of holoprosencephaly. There is


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The ventricle measurement at the atrium is

greater than 10mm. Look for other CNS

abnormalities.Can be associated with

Trisomy 21,18 ,13 and triploidy.

a large single ventricle. No cortical mantle

and fused thalami.Can be associated with

Trisomy 13 and 18.

'Banana Cerebellum' sign

Associated with spina bifida (secondary to

cord tethering).


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"Lemon Sign" is inward scalloping of the

frontal bones and is associated with "open"

spina bifida and the Chiari II malformation

Myelomeningocele.Sagittal ultrasound

image demonstrates the break in the skin

line.The myelomeningocele sac can be


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detected on sagittal or transverse views.

Even in the absence of a sac,

myelomeningocele is suggested by a defect

in the normal smooth dorsal skin line and

splayed posterior ossification centres on the

transaxial image

Cleft Palate

The fronto-nasal process of the face fails to

unite with the maxillary prominences at

approximtely 3-4 weeks of gestation.It can

be isolated on the left or right of midline or

may be associated with other

malformations if midline.It can be

associated with Trisomy 13 and 18.Imaging

the face in a 2D coronal view initially willdiagnose this and then a 3D surface

rendered image will give not only the

sonographer/doctor a better diagnosis but

also give the parents improved visualisation

so as to prepare them for the baby's

appearance.


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Cystic AdenomatoidMalformation (CAM)

Cystic Adenomatoid Malformation is seen

as hyperechogenic lungs due to an

overgrowth of the terminal bronchioles.

Fluid accumulates in the small cystic spaces.

This echogenic segment of lung is

pathognemonic for a CAM lesion.

CAM or CCAM

Also called a Congenital Cystic

Adenomatoid Malformation.

This example is a large echogenic segment


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with no discernable cysts (microscopic,

therefore Type III).

Omphalocele

Omphalocele There is herniated abdominal

contents through the anterior abdominal

wall at the cord insertion site.Be careful

when diagnosing this as the viscera isnormally herniated into the cord before 11

and half weeks gestation.Most commonly

associated with amniotic band

syndrome,trisomy 18 and 13 and umbilical

hernia.

Gastroschisis

Gastroschisis.This is an abdominal wall

defect which does not involve the cord

insertion.


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Hydrops

Hydrops 2 Types immune and non immune.

Non immune is the more common seen in

the 21st Century since anti D injections.

Associated with ascites, pleural,pericardial

effusions and/or subcutaneous

oedema.Placental oedema is seen in the late

3rd Trimester.

Hydrops

Hydrops


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Megacystis

Megacystis Commonly caused by posterior

urethral valves in a male.In a female it is a

cloacal anomaly. In both genders it can be

due to urethral atresia and megacystis-

microcoloc-intestinal hypoperistalsis

syndrome.

Pyelectasis

Pyelectasis. Criteria at the 18 to 19week

scan is 0-5mm renal pelvis measurement

taken in a transverse plane. In 3rd trimester

up to 10mm is within normal limits.A

detailed examination to check for other

abnormalities and causes should be

performed with serial scanning to see if it

resolves.Can be associated with Trisomies13,18,21 and Turners Syndrome.


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A ureterocele is causing pyelectasis and

ureter dilatation in a male foetus


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Neuroblastoma. This tumour has a complex

sonographic appearance with a solid and a

cystic component. It sits on the left side

medial and superior to the kidney

retroperitoneally.Neuroblastomas can

displace the Inferior Vena cava anteriorly

and can be found in the neck, chest or in a


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paravertebral intra-abdominal location.

They are malignant.They may not be seen

in the morphology scan, but can develop in

the 3rd Trimester. The Mother does not

always have any symptoms.

Clubfoot

When imaging look at the tib/fib when it is

true AP as you should NOT see the foot

from this view.(Show normal v's

abnormal)It may be on one side only.Trisomy 18,athryogryposis,neural tube

defect or it can be isolated.


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Bowing of Bones

Bowing of bones Osteogenesis

imperfecta,camptomelic

dysplasia,thanatophoric dysplasia

Limb shortening

Limb shortening If there is a suspicion of

the limbs being shortened then

measurements must include not only the

femora and humeri but also the radius/ulna

and tibia/fibula which should also be

graphed. Can be due to

Achondrogenesis,Osteogenesis imperfecta

and thanatophoric dysplasia

Hyperflexed wrist:

The joints should be seen to flex and extend.


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Maintained forced flexion suggests

dysplasia of some kind and warrants

further investigation.

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What Are Obstetric Ultrasound Scans