First Trimester Ultrasound

1. First Trimester Ultrasound Maternal and Child Health Tutorial Sarah Gopman, MD Updated 8-29-04

2. Learning Objectives • Understand basic scientific principles of ultrasound • Learn a systematic approach to the first trimester ultrasound exam • Apply findings of first trimester ultrasound to pregnancy dating • Distinguish between normal and abnormal pregnancies • Understand applications of ultrasound in medical and surgical termination of pregnancy

3. Principles of Ultrasound • Bodily structures reflect sound waves differently depending on the density of the tissue • Ultrasound uses high-frequency waves (2–10 MHz) to visualize internal structures • The ultrasound transducer both transmits and receives sound waves • Sound waves reflected off internal structures are converted into an electrical signal, which is used to generate a two-dimensional image on an oscilloscope screen

4. Principles of Ultrasound • The higher the frequency, the better the resolution (i.e. the picture is clearer) • However, higher frequency results in less penetration and therefore a shorter field of view • This is the principle by which endovaginal US works—the transducer is closer to the object being visualized, compensating for the decreased penetration and conveying the benefits of better resolution

5. Ultrasound terms • Isoechoic—same texture as myometrium • Hyperechoic—appears whiter than myometrium • Hypoechoic—appears darker than myometrium • Anechoic—appears black, similar to urine in bladder (fluid)

6. Utility of routine early ultrasound • Diagnose early pregnancy and establish accurate dating • Verify embryonic/fetal life • Determine intrauterine vs extrauterine pregnancy • Evaluate number of gestations • Determine uterine characteristics • Determine adnexal characteristics

7. When early US is especially important • Abdominal pain or bleeding in early pregnancy • Hx of prior ectopic pregnancy • Determine delivery date for repeat c/s • Potential to induce labor • Hx of pre-eclampsia with prior pregnancy • Diabetes • Other maternal medical condition, such as SLE, etc • Patient considering first trimester genetic screening • Patient considering termination of pregnancy

8. Transabdominal US: Advantages and Disadvantages • Advantages • Provides a “panoramic” pelvic view • Noninvasive • Easy to learn • Simple when findings are present • Disadvantages • Often requires distended bladder for visualization • Expense • Unable to detect pregnancies less than 6 weeks’ gestation

9. Endovaginal US: Advantages and Disadvantages • Advantages • Detects earlier pregnancies • Readily combined with pelvic exam • Does not need a distended bladder • Easy skill to learn • Disadvantages • Expense of examination • Invasive • Potential for infection

10. Longitudinal/sagittal cervix in the upper right region fundus in mid left region bladder in upper left region spine to right Transverse (90 degrees counter-clockwise from longitudinal) patient’s right side on left of screen patient’s left side on right of screen Screen Image—Endovaginal

11. True relationship b/w patient and transducer

12. Relationships on the screen

13. Uterus, endometrial stripe, cervix

14. Pregnancy Testing • Implantation occurs 5-7 days after fertilization • Trophoblastic cells begin to produce hCG • Urine pregnancy test positive at time of missed menses • Can detect levels of 25 mIU/ml • Serial quantitative hCG testing best performed through same laboratory

15. Structures of 1st Trimester Pregnancy • Gestational sac • Yolk sac • Embryo/fetus • Presence of cardiac activity

16. Gestational sac • Earliest visualized structure • Seen as early as 30 days from LMP via endovaginal ultrasound • Eccentric to endometrial stripe • Choriodecidual “double ring” reaction • increased blood flow and endometrial thickening • not seen if pseudosac of ectopic • Chorionic sac contains yolk sac; amniotic sac contains embryo • Amnion usually fuses with the chorion at 8-10 weeks (obliterates yolk sac)

17. Eccentric implantation of GS

18. Eccentric implantation of GS

19. True GS vs. pseudosac

20. Beak Sign • The intrauterine fluid collection (or sac) shows a small “beak” that connects with or points toward the uterine cavity line • Highly suggestive of fluid collection or pseudo-gestational sac (Yeh, 1999)

21. Double Decidual Ring

22. Yolk Sac • Seen at 5-6 weeks LMP, persists to 11-12 weeks • Circular, central anechoic area, measures 3-8 mm • Larger than the embryo initially • Identifies an intrauterine pregnancy • 2/3 of pregnancies with yolk sacs continue • Connected to embryo but outside amniotic sac • Yolk sac excludes pseudogestational sac

23. Yolk sac—confirmation of IUP

24. Embryo • Cardiac activity in 6th week when embryo is 1-3 mm • 95% of pregnancies with cardiac activity continue • Crown rump length (CRL) provides most accurate dating • Limb buds observed at 8 weeks and terminology changes from embryo to fetus

25. Estimating Gestational Age • Measurements • embryo (crown rump length) • if embryo not measurable, use gestational sac • Gestational sac measurement • Inner margins of sac in 2 or 3 dimensions • Estimated age (days) = 30 + mean gs (mm) • Gestational sac grows at 1 mm/day • Can be visualized as early as hcg of 500 mIU/mL

26. Discriminatory Level • hCG level at which all normal gestational sacs should be visualized • Value for transabdominal ultrasound = 3600 mIU/ml • Value for endovaginal ultrasound = 1500–2000 mIU/ml

27. Expected hCG levels

28. Estimating GA by embryo size Embryonic length (crown-rump length=CRL) measurement Most accurate ultrasound estimate of GA Avoid limbs and yolk sac CRL formula (Goldstein): estimated age (days) = CRL (mm) + 42

29. Seven week IUP

30. First Trimester IUP—magnified view

31. Later First Trimester IUP

32. END OF PART ONE

33. First Trimester US: Part TwoReview of Learning Objectives • Understand basic scientific principles of ultrasound • Learn a systematic approach to the first trimester ultrasound exam • Apply findings of first trimester ultrasound to pregnancy dating • Distinguish between normal and abnormal pregnancies • Understand applications of ultrasound in medical and surgical termination of pregnancy

34. Later First Trimester IUP

35. Systematic Exam • Longitudinal view (transducer button upright) • Go from one side to the other • Scan uterus and cul de sac • Measure pregnancy if present—magnifystructure being measured! • Transverse view (transducer button to left) • Go from top to bottom • Scan uterus and ovaries

36. Requirements for “standard exam” in first trimester, per AIUM • Evaluate uterus and adnexae for gest. sac • Crown-rump length of embryo, if present • If no embryo, look for yolk sac and measure gest. sac • Gest. sac w/o embryo/yolk sac is not proven IUP • Document presence of embryonic heart motion (seen if CRL > or = 5mm) and fetal number

37. Ultrasound of Ovaries • Not always seen • Found in transverse view moving probe up and down • medial to iliac vessels • follicles remain round on all views, whereas blood vessels become tube-like • Can measure ovarian size in three dimensions

38. Normal ovary with follicles

39. Ovarian cysts • Size and consistency change over time • Functional Cysts • anechoic; sharp, thin walls • Ignore if <3 cm diameter • Monitor if >3 cm diameter • If >6 cm diameter for 6-12 monthsrefer • Corpus luteum cysts • Can reach 5 cm diameter • Thin-walled • Can be seen in various states of resolution • Regression by 10 weeks GA

40. Corpus luteum cyst

41. Abnormal pregnancy • Estimated that only half of fertilized eggs become clinical pregnancies • One fourth of clinical pregnancies will have threatened abortion—one-half of these will spontaneously abort • Cardiac activity in a threatened abortion has a better prognosis

42. US findings and miscarriage • Empty gestational sac • Deformed gestational sac • Gestational sac low in the uterus • Gestational sac or embryo which fails to grow

43. Abnormal yolk sac • >10 mm yolk sac diameter • Yolk sac not present in 29 mm gestational sac • Yolk sac not round

44. Abnormal embryo • No cardiac activity when embryo is 5 mm = poor prognosis • Gestational sac >25 mm and no embryo = poor prognosis

45. Ectopic pregnancy • 0.5-2% of all pregnancies and increasing • Fewer maternal deaths due to earlier diagnosis • Location • >95% in tube • 1-3% in cornual,cervical and abdominal locations • Heterotopic pregnancies occur at 1/30,000 but higher in assisted pregnancy

46. Ectopic Pregnancy: Risk Factors • Previous ectopic pregnancy • Tubal surgry of any kind • Known tubal pathology (sequela of PID) • DES exposure

48. Cervical Ectopic Pregnancy • Occurrence is less than 1% of ectopic pregnancies. • Risk factors are multiparity, prior surgical abortion, and instrumentation of cervix. • Differential diagnosis is incomplete spontaneous abortion. • Morbidity and mortality increased due to profuse bleeding.

49. Interstitial (Cornual) Pregnancy • Occurrence is 2-4% of all ectopic pregnancies • Myometrium surrounds a portion of the growing gestational sac, allowing it to expand painlessly for a relatively long time. • Increased morbidity and mortality because of later presentation and massive hemorrhage.

50. Findings in Ectopic Pregnancy • Diagnostic: sac, fetus, and fetal cardiac activity in the adnexa • Suggestive: • Absence of intrauterine GS when hCG above discriminatory zone • Complex adnexal mass and fluid in the cul-de-sac • Failure of hCG to double in 2 days • About 10% of ectopic gestations will have have hCG doubling times in the normal range • 12% of IUPs will have hCG’s that fail to double in the normal range(Romero et al., 1986 and Kadar et al, 1981).