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 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 1
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 2
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 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 3
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 Assessment of fetal anatomy Advantages Early detection and exclusion of major anomalies 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 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 4
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) 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 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 5
Week 4: gestational sac only; 2-5 MSD Week 4: gestational sac only Week 5: GS + yolk sac; ~ 4 mm; GS ~ 6 MSD 6
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 7
Week 7: identify cord; determine cranial and caudal ends; primitive ventricular system Week 7: rhombencephalon and limb buds Week 8: 4 sequential sonolucencies; observe upper and lower limb buds; spine and stomach 8
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: 10 th to the 14 th weeks Few new structures appear Can perform a limited anatomic 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 9
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Intracranial anatomy 11
First trimester screening Nuchal translucency + nasal bone 12
Absent nasal bone Increased nuchal translucency Gender determination 13
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