First Trimester Fetal Ultrasound Martha Decker, MD. First trimester. Purpose of first trimester ultrasound
First Trimester Fetal Ultrasound Martha Decker, MD
First trimester • Definition (ISUOG) • Stage of pregnancy starting from time when viability can be...
First Trimester Fetal Ultrasound Martha 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 • Embryo: before 10 weeks • Fetus: after 10 weeks; organogenesis essentially complete
Wake Forest Baptist Medical Center
Purpose of first trimester ultrasound • Provide information to provide optimized antenatal care with best possible outcomes for mother and fetus • Confirm viability • Establish gestational age • Determine number of fetuses; if multiples determine chorionicity/amnionicity • Detect gross structural abnormalities • Measure nuchal translucency Wake Forest Baptist Medical Center
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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
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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 Wake Forest Baptist Medical Center
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
Wake Forest Baptist Medical Center
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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 neutral but t l position; iti clear l end d points i t off 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
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Neck to rump length 6 to 9 weeks Wake Forest Baptist Medical Center
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 • Can use singleton nomograms for multiples • General rule: use CRL for GA when < 84 mm; • then use HC followed by BPD
Wake Forest Baptist Medical Center
Assessment of fetal anatomy • Advantages • Early detection and exclusion of major anomalies • Early reassurance to at at-risk 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 Wake Forest Baptist Medical Center
Assessment of fetal anatomy • 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 Wake Forest Baptist Medical Center
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Assessment of fetal anatomy • Neck: NT measurement; hygromas and jugular lymph sacs • Spine: longitudinal and transverse views; 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) Wake Forest Baptist Medical Center
Assessment of fetal anatomy • 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
Wake Forest Baptist Medical Center
Assessment of fetal anatomy • Abdominal wall: > 12 weeks, assess cord insertion site (physiological umbilical hernia seen up to 11 weeks); gastroschisis and 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 Wake Forest Baptist Medical Center
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Week 4: gestational sac only; 2-5 MSD
Wake Forest Baptist Medical Center
Week 4: gestational sac only
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Week 5: GS + yolk sac; ~ 4 mm; GS ~ 6 MSD
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End of week 5: GS + YS + embryo; 2-3 mms
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Week 6: embryo 4-9 mm; cardiac activity by 4 mm
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M-mode for heart rate
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Week 7: identify cord; determine cranial and caudal ends; primitive ventricular system
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Week 7: rhombencephalon and limb buds
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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
Wake Forest Baptist Medical Center
Fetus: 10th to the 14th weeks • Few new structures appear • Can perform a limited anatomic survey: survey brain, heart, limbs, stomach, bladder, kidneys, face 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 Wake Forest Baptist Medical Center
