first trimester fetal ultrasound.pptx [read-only] 4: gestational sac only; 2-5 msd wake forest...

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First Trimester Fetal UltrasoundMartha Decker, MD

First trimester

• Definition (ISUOG)• Stage of pregnancy starting from time when

viability can be confirmed (IUP with cardiac activity) up to 13+6 weeks gestation

Wake Forest Baptist Medical Center

activity) up to 13+6 weeks gestation• Embryo: before 10 weeks• Fetus: after 10 weeks; organogenesis

essentially complete

Purpose of first trimester ultrasound

• Provide information to provide optimized antenatal care with best possible outcomes for mother and fetus

Confirm viability


• Confirm viability• Establish gestational age• Determine number of fetuses; if multiples -

determine chorionicity/amnionicity• Detect gross structural abnormalities• Measure nuchal translucency

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Safety during first trimester ultrasound

• Principle: scan the shortest possible time using the lowest possible power output necessary

• B-mode and M-mode OK


• Doppler (greater energy output) used only if clinically indicated

Viability

• Embryo with cardiac activity• Can be visualized at ~37 days gestation when

heart tube begins to beat


• Fetal pole measures ~ 2 mm or more• 5-10% will not have cardiac activity between 2

and 4 mm• Once viable, embryo increases in length

1mm/day

Early pregnancy measurements

• Gestational sac visible starting at ~ 35 days• Mean sac diameter (MSD): average of 3

orthogonal measurements of fluid-filled space


• CRL of embryo more accurate estimation of GA than MSD

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First trimester measurements• Crown rump length (CRL)

• Ideally: midline sagittal section of whole embryo; image magnifies to fill most of screen horizontally; fetus not flexed or hyperextended b t t l iti l d i t f


but neutral position; clear end points of crown and rump; avoid inclusion of yolk sac; 6-9 weeks measuring neck-rump length; at 10 weeks, want to have fluid between chin and chest


Neck to rump length 6 to 9 weeks

Crown to rump length 10 to 14 weeks


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Assessment of gestational age

• CRL most accurate determination GA in within 5 days either way

• Optimal time for GA is between 8 and 13+ weeks


weeks• Can use singleton nomograms for multiples• General rule: use CRL for GA when < 84 mm;

• then use HC followed by BPD

Assessment of fetal anatomy

• Advantages• Early detection and exclusion of major

anomaliesEarly reassurance to at risk mothers


• Early reassurance to at-risk mothers• Earlier genetic diagnoses• Safer pregnancy termination

• Limitations: trained and experienced scanners and late development of some structures like hypoplastic heart and cerebellar vermis


• Head• Ossification by 11 weeks (look in 2 planes)• From 11 to 13+ weeks, dominated by large


lateral ventricles filled with choroid plexuses in posterior two thirds ie. “butterfly” sign

• Mantle: very thin and shouldn’t be mistaken for hydrocephalus

• Try to visualize lens, orbits, profile, NB, mandible +/- mouth and lips

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• Neck: NT measurement; hygromas and jugular lymph sacs

• Spine: longitudinal and transverse views; attempt made to show intact overlying skin


attempt made to show intact overlying skin• Thorax: effusions; cystic or solid masses;

ensure that stomach and liver are in abdomen• Heart: location in left chest; no Doppler during

routine scanning (use M-mode or clip for FHR)


• Abdomen: 11 to 13+ weeks• Stomach and bladder – only hypoechoic

structures


• Stomach on left• Kidneys – slightly echogenic paraspinal

structures (tough)• Bladder visible by 12 weeks


• Abdominal wall: > 12 weeks, assess cord insertion site (physiological umbilical hernia seen up to 11 weeks); gastroschisis and omphalocele can be diagnosed


omphalocele can be diagnosed• Limbs: 11 to 13+ weeks - each bony segment

can be identified; hands by 11 weeks• Genitalia: orientation of genital tubercle• Umbilical cord: brief evaluation of bladder

region with Doppler can confirm 1 or 2 arteries

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Week 4: gestational sac only; 2-5 MSD


Week 4: gestational sac only


Week 5: GS + yolk sac; ~ 4 mm; GS ~ 6 MSD


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End of week 5: GS + YS + embryo; 2-3 mms


Week 6: embryo 4-9 mm; cardiac activity by 4 mm


M-mode for heart rate


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Week 7: identify cord; determine cranial andcaudal ends; primitive ventricular system


Week 7: rhombencephalon and limb buds


Week 8: 4 sequential sonolucencies; observe upper and lower limb buds; spine and stomach


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Week 9: embryo begins to unfold; visualize CI, physiologic midgut herniation (8.5 to 10.5 weeks), long bones, legs, feet, arms and fingers


Fetus: 10th to the 14th weeks

• Few new structures appear• Can perform a limited anatomic surveylimited anatomic survey: brain,

heart, limbs, stomach, bladder, kidneys, face and gender


and gender• Measure several parts of the fetus• Start seeing details of the heart; can perform

fetal “ECHO” at 13 weeks• Measure nuchal translucency and assess nasal

bone for first trimester screening

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Intracranial anatomy


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First trimester screening


Nuchal translucency + nasal bone


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Absent nasal bone


Increased nuchal translucency


Gender determination


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first trimester fetal ultrasound.pptx [read-only] 4: gestational sac only; 2-5 msd wake forest...

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