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SON5023 Obstetrics > Topic 3 - 11-14 weeks scan > Flashcards

Topic 3 - 11-14 weeks scan Flashcards

1
Q

What is gastrulation?

A

bilaminar disk to a trilaminar disk with the three primary germ cell layers: ectoderm, mesoderm, and endoderm

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2
Q

What is neurulation?

A

The formation of the neural plate and its closure to form the neural tube
process begins in the fifth week in the thoracic region and extends caudally and cranially, resulting in complete closure by the end of the sixth week (day 42).
Failure of closure of the neural tube results in neural tube defects.

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3
Q

When does the primitive heart form?

A

Week 5. two cardiac tubes develop from splanchnic mesodermal cells. By the end of the fifth week, these tubes begin to pump

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4
Q

What are the two things that happen in week six of development?

A

By the end of the sixth week, blood flow is unidirectional

The primitive gut forms

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5
Q

What are the three things that happen in week 8?

A

The midgut herniates into the umbilical cord from week 8 through the end of week 12
The rectum separates from the urogenital sinus by the end of week 8, and the anal membrane perforates by the end of week 10.
The metanephros, or primitive kidneys, ascend from the pelvis, starting at approximately week 8, but do not reach their adult position until week 11.

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6
Q

When do all internal and external structures appear in the adult form?

A

all internal and external structures appear in the adult form during the embryonic period, which ends at 10 menstrual weeks

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7
Q

When does the heart attain its definitive form?

A

End of week 8

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8
Q

When does the peripheral vascular system develop?

A

Week 10

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9
Q

When are Limbs are formed with separate fingers and toes?

A

Week 10

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10
Q

By what time have all congenital malformations except abnormalities of the genitalia happened?

A

Week 10

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11
Q

By hat time have genitalia formed?

A

External genitalia are still in a sexless state at the end of week 10 and do not reach mature fetal form until the end of week 14.

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12
Q

What happens to he CRL between 11-14 weeks?

A

It doubles

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13
Q

On sonography what is visible at week 5?

A

Gestational sac and yolk sac

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14
Q

On sonography what is visible at week 6?

A

Fetus on the edge of the yolk sac.

CRL 2mm

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15
Q

On sonography what is visible at week 7?

A

Fetus separate from the yolk sac

CRL 10mm

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16
Q

On sonography what is visible at week 8?

A

Fetal movements, limb buds, rhombencephalon, amniotic membrane
CRL 18mm

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17
Q

On sonography what is visible at week 9?

A

Brain division, Limbs visible, fetal movements seen.

CRL 25mm

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18
Q

On sonography what is visible at week 10?

A

Profile seen, physiological exomphalos

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19
Q

On sonography what is visible at week 11?

A

NT measureable, hands and feet seen, brain, choroid, lat vents
CRL 45mm

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20
Q

On sonography what is visible at week 12?

A

NT measurable, exomphalos gone, abdominal wall in tact, fingers visible, bladder fills, stomach visible
CRL 60mm

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21
Q

On sonography what is visible at week 13?

A

As for week 12

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22
Q

On sonography what is visible at week 14?

A

Heart detail visible

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23
Q

What are the three major points that should be remembered when considering first trimester pathology?

A

1) normal embryologic/fetal development may mimic pathology
2) abnormal embryos/fetuses may appear normal early in pregnancy
3) discrepancies between dates and embryo size may be the only visible manifestation of pathology in some first-trimester examinations.

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24
Q

What is the rhombencephalon?

A

During the sixth week, three primary brain vesicles form: the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain).
earliest cystic structure seen in the posterior aspect of the embryonic head at 6 to 8 weeks’ gestation represents the normal embryonic cystic rhombencephalon
It later forms the normal fourth ventricle

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25
Q

Why is recognising the rhombencephalon important?

A

should not be mistaken for a posterior fossa cyst of pathologic importance

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26
Q

How do the lateral ventricles appear during development?

A

9 weeks, the lateral ventricles identified as two small cystic spaces
11 weeks more evident
12 weeks the lateral ventricles extend almost to the inner table of the skull only a small rim of cerebral cortex can be demonstrated to surround them.
choroid plexus is echogenic and fills the lateral ventricles completely except for the frontal horns

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27
Q

What is the normal process of physiological anterior abdominal wall herniation?

A

midgut normally herniates into the umbilical cord at the beginning of the eighth week
midgut rotates 90 degrees counter clockwise and then returns to the abdomen during the 12th week.

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28
Q

What does a physiological exomphalos look like on sonography?

A

herniated bowel appears as a small, echogenic mass (6-9 mm) protruding into the cord at approximately 8 weeks (CRL, 17-20 mm)
Follow-up examinations reveal reduction of the hernia between 10 and 12 weeks
20% of normal pregnancies, the herniated bowel may still be found outside the fetal abdomen at 12 weeks.

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29
Q

At what time is pregnancy first detectable?

A

very early pregnancy is detectable by a gestational sac from 35 days (5 weeks)

30
Q

What is the gold standard for gestational age estimation?

A

The crown-rump length between 8-14 weeks

31
Q

What are the uses of ultrasound in the first trimester?

A

Identification of missed abortion, ectopic pregnancy, and C-section scar pregnancies.
Early identification of multiple gestation and correct determination of chorionicity.
Accurate dating means optimal assessment of fetal growth later in pregnancy, appropriate obstetric care in general and management of preterm or post-term pregnancies in particular.
Screening for fetal abnormalities
gives parents the opportunity to think about all their options if a major abnormality is found

32
Q

Comment on the precision and consistency of fetal organ visualisation in early pregnancy

A

Due to improvement in ultrasound technology fetal organs can be accurately assessed in early pregnancy.
dependent on equipment, sonographer experience and gestation
It is important not to misinterpret normal physiological changes of the fetus.
normal distribution of growth is narrow in early pregnancy and widens significantly as growth progresses (and its rate slows).
It is because of this that CRL is an accurate predictor of gestational age and why biometrics such as BPD, AC, FL are used to assess for things such as IUGR rather than provide a gestational age.

33
Q

What is the nuchal translucency?

A

subcutaneous collection of fluid in the fetal neck between the skin and the cervical spine
In normal fetuses NT thicknees increases with CRL
determines the risk of chromosomal abnormality
the best single indicator of aneuploidy, particularly Down Syndrome, in an otherwise normal fetus.
It is not an abnormality but an indicator of risk
Its real value as a measurement is only realised when it is combined with first trimester maternal serum and a combined risk calculated.

34
Q

When is an NT measurement performed?

A

performed when a woman presents in this 11-14 week window for screening of Down Syndrome.
The role of this measurement will depend on whether the patient has had NIPT

35
Q

What is NIPT (noninvasive prenatal testing)?

A

NIPT is a maternal blood test that looks for fetal DNA in the maternal plasma (cell-free DNA).
allows a simple maternal blood test to detect Down syndrome in a fetus with a very high level of accuracy (at least 99.5% with a false-positive rate of 0.2%).
Can be undertaken from 10 weeks gestation

36
Q

Why is an ultrasound still performed after NIPT?

A 
To ensure correct dating
Diagnosis of multiple pregnancies and chorionicity
Anatomy assessment to detect major abnormalities:
Acrania
Alobar holoprosencephaly
Exomphalos
Gastroschisis
Megacystitis
Body stalk anomalies
37
Q

What are the benefits of current screening?

A

first trimester combined screening (US NT and Blood) has a detection rate for Down syndrome of 90% and a false-positive rate of 3%
provides more information than only the risk of the three most common trisomies
NIPT is not a replacement for first-trimester ultrasound and blood testing at 11–13 weeks.
patient will often incur an out-of-pocket cost for first trimester combined screening, it is substantially less than that for NIPT at the present time.

38
Q

What are the limitations of NIPT?

A

Cost (500-1400)
Cannot replace US and blood screening
It is unlikely to give a false-positive result (0.2%) but all positive results need to be confirmed by an invasive test (amniocentesis or chorionic villus sampling).
There is a test failure rate of up to 4% (this is higher as body mass index increases: the test failure rate is likely to be 50% at a maternal weight of 160 kg).

39
Q

What are the benefits of NIPT?

A

The test is very accurate for detection of Down syndrome (sensitivity 99.5%) and Edward syndrome (99%). It is less accurate for the detection of Patau syndrome (79–92%).
NIPT can also test for fetal sex and some sex chromosome abnormalities.

40
Q

Comment on false positives during aneuploidy screening

A

With each test performed there is an increasing false positive rate.
As such an integrated screening program which establishes risk at only one point is generally implemented.

41
Q

briefly outline the protocol for attaining a NT measurement

A

CRL is between 45 and 84mm
The foetus is in the true sagittal plane, and horizontal.
Ideally, the mid-sagittal view of the face should be obtained as defined by the presence of then echogenic tip of the nose and rectangular shape of the palate anteriorly, and the translucent diencephalon in the centre with the nuchal membrane posteriorly
The foetus is in a neutral position, with the head in line with the spine and not hyper-flexed or hyper-extended.
Hyper-flexed = NT is falsely decreased
Hyper-extended = NT is falsely increased
The image should be optimised so that the foetus is magnified, and the head and thorax occupy the whole image.
The widest part of the NT is measured.
Measurements taken on the inner border of the horizontal line (the sharpest part of the bone)
Multiple measurements should be taken with the widest measurement recorded

42
Q

How should you optimise machine setting for NT measurement?

A
  1. Avoid post freeze zoom.
  2. Reduce the depth.
  3. Reduce the sector width.
  4. Move the focus to the level of the NT.
  5. Reduce the gain to reduce the fuzzy edges.
  6. Reduce the dynamic range.
  7. Review use of harmonics.
43
Q

Interpret an increased NT

A

Increased NT refers to a measurement above the 95th centile
After 14 weeks, increased NT usually resolves, but in some cases it evolves into nuchal edema or cystic hygromas
A fetal NT above 3.5 mm is found in about 1% of pregnancies.
The risk of major chromosomal abnormalities is very high

44
Q

How do you deal with a situation where cord crosses the nuchal translucency and causes a focal narrowing?

A

a measurement of the maximum size on either side of the narrowing should be recorded. The average should be calculated as the final measurement.

45
Q

After you performed a NT measurement, the patient was told that the result of the test was “very good”. The patient is very happy and replies that it is a great relief to know that the baby does not have Down Syndrome. Comment on this reply.

A

Measurement of the nuchal translucency is a screening test and not a diagnostic test.
Down Syndrome can only be excluded with certainty by karyotyping. This requires invasive testing such as CVS or amniocentesis. The nuchal translucency screening provides a risk estimate. The test result is said to be good, when the estimated risk of Down Syndrome is very low, but never zero. When the calculated risk for trisomy 21 is high (usually higher than 1/300), the patient will be offered invasive testing.

46
Q

What are the possible mechanisms that cause and increased NT?

A

There are a range of conditions that cause Nuchal Translucency to be thick suggesting there is not just one underlying mechanism. Possible mechanisms may include;
cardiac dysfunction in association with abnormalities of the heart and great arteries
venous congestion in the head and neck
altered composition of the extracellular matrix
failure of lymphatic drainage caused by abnormal or delayed development of the lymphatic system or impaired fetal movements
fetal anemia or hypoproteinemia
congenital infection.

47
Q

Comment on increased NT and chromosomal defects

A

The prevalence of chromosomal defects increases exponentially with NT thickness.

48
Q

Comment on increased NT and fetal death

A

In chromosomally normal fetuses, the prevalence of fetal death increases exponentially with NT thickness.

49
Q

Comment on increased NT and major fetal abnormalities

A

The prevalence of major fetal abnormalities in chromosomally normal fetuses increases with NT thickness
Major fetal abnormalities are defined as those requiring medical and/or surgical treatment or conditions associated with mental handicap.

50
Q

Comment on increased NT and developmental delay

A

Not been studied extensively.

No significant data to correlate

51
Q

Comment on increased NT and cardiac defects

A

high association between increased NT and cardiac defects
increased nuchal translucency constitutes an indication for specialist fetal echocardiography
chromosomally normal fetuses, the most common abnormalities were narrowing of the aorta at the level of the isthmus and septal or valvular defects

52
Q

Comment on increased NT and cardiac defects in chromosomally abnormal fetuses

A

Trisomy 18 was associated with ventricular septal defects and/or polyvalvular abnormalities
trisomy 13, there were atrioventricular or ventricular septal defects, valvular abnormalities, and either narrowing of the isthmus or truncus arteriosus
Turner syndrome was associated with severe narrowing of the whole aortic arch
In all 4 groups of chromosomally abnormal fetuses, the aortic isthmus was significantly narrower than in normal fetuses, and the degree of narrowing was significantly greater in fetuses with increased NT thickness.

53
Q

Comment on increased NT and body stalk anomaly

A

is a rare, severe birth defect involving a large abdominal wall defect and a group of associated malformations. The condition is sometimes referred to by other names, including limb-body wall complex, amniotic band disruption complex, and amnion rupture sequence
lethal
sporadic
fetal NT was increased in 84% of the fetuses, the karyotype was normal

54
Q

Comment on increased NT and CDH (congenital diaphragmatic hernia)

A

Increased NT thickness is present in about 40% of fetuses with diaphragmatic hernia
sporadic defect
In 30% of affected fetuses, there are associated chromosomal abnormalities or other defects
In isolated diaphragmatic hernia, survival after postnatal surgery is about 50%
the remainder dies because of pulmonary hypoplasia and pulmonary hypertension

55
Q

Comment on increased NT and exomphalos

A

Increased NT is observed in about 85% of chromosomally abnormal and 40% of chromosomally normal fetuses with exomphalos.

56
Q

Comment on increased NT and megacystitis

A

associated with increased NT
was observed in about 75% of those with chromosomal abnormalities, mainly trisomy 13
in about 30% of those with normal karyotype
Fetal megacystis at 11 to 13 + 6 weeks of gestation, defined by a longitudinal bladder diameter of 7 mm or more,

57
Q

Comment on increased NT and TTTS

A

In monochorionic twins, discordancy for increased NT is an early marker of twin-to-twin transfusion syndrome (TTTS).

58
Q

What are the most commonly used first trimester ultrasound markers used with nuchal translucency to gauge risk in australia

A

Nasal Bone
Ductus Venosus
Tricuspid Regurgitation

59
Q

Why are the use of extra ultrasound markers being increased

A

Detection rates of Down syndrome of 80%, 87%, and 94% are reported when using one, two, or three additional ultrasound markers while maintaining the FPR at 3%

60
Q

Why is the nasal bone assessed?

A

small nose is a common with trisomy 21.
nasal bone abnormalities are significantly more common in trisomy 21 fetuses than in euploid fetuses
abnormalities include both nasal bone absence and short nasal bone length.

61
Q

What is the criteria for measuring nasal bone?

A

Criteria for imaging is similar to NT measurement

Plus 90- degree angle of insonation of NB 11-14 weeks

62
Q

How does the nasal bone appear on sonography during its assessment?

A

If the NB is present, the view will demonstrate 2 echogenic lines which represent the skin on the surface of the nasal bone and the bone itself
The 2 parallel lines representing the skin over the nasal bridge and the nasal bone compose the so-called ‘‘equal sign.’’
The line representing the nasal bone is thicker and more echogenic than the skin and usually contains a highly echogenic center.

63
Q

What should you do if the NB is absent between 11 and 11+ 6 weeks?

A

repeat examination 1 week later should occur as it may just be too early

64
Q

What is the role of the ductus venosus and how does it function?

A

50% of the oxygenated blood brought back from the placenta by the umbilical vein courses through the ductus venosus
This empties into the IVC near the right atrium
On pulse Doppler, this demonstrates forward flow through the cardiac cycle. Blood flow significantly decreases during an atrial contraction which is called the a-wave

65
Q

How might the ductus venosus appear in fetuses with trisomy 21?

A

A-wave anomalies are seen in 80% of fetuses with trisomy 21

Absent or reversed

66
Q

What does tricuspid regurgitation signify?

A

Abnormalities of the great vessels.

67
Q

What is FMF?

A

Frontomaxillary facial angle

fronto-maxillary angle measurements have been reported to be a marker for aneuploidy

68
Q

What are the criteria for measuring FMF angle?

A

Foetus needs to be in profile facing the transducer
Must be mid-sagittal plane
Hard palate should be seen as a rough trapezoidal structure
Skin over the forehead should be seen separately from the metopic suture
Intracranial landmarks that are seen in this view are the thalamus, pons and medulla oblongata

69
Q

How do you measure FMF angle?

A

The FMF angle:
First ray is drawn along the upper edge of the hard palate
Apex is at the anterior edge of the maxilla
Second ray runs from the apex upwards resting on the echogenic line beneath the skin (metopic suture)

70
Q

What are the normal limits of FMF angle?

A

Angle is increased in trisomy 21
The FMF angle was above 85◦ in 69% of the trisomy 21 fetuses and in 5% of the normal fetuses
Normal is classified as 85 degrees +/- 10 degrees

SON5023 Obstetrics (9 decks)

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