Principles and Applications of Ultrasound to Obstetrics

1. Principles and Applications of Ultrasound to Obstetrics Honor M. Wolfe

2. What is the accuracy of ultrasound in the assessment of gestational age?

3. GA Assessment Accuracy∞ 1/Gestational Age

5. Gestational age accuracy 1st trimester + 1 week 2nd trimester + 2 weeks 3rd trimester + 3 weeks

6. First Trimester: CRL 5-12 weeks gestation < 10 wks + 3-5 days > 10 wks less accurate variable position/flexion 5-7 days

7. 2nd and 3rd trimester Accuracy of GA estimates increases as more variables are measured. - Composite estimate of: Biparietal diameter Head circumference Femur length Abdominal circumference

8. Does maternal BMI impact ultrasound and if so how and why?

9. Physics • High frequency sound waves • > 20,000 cycles/second • Frequency • Number of waves per unit time • Expressed as hertz (Hz) • Diagnostic ultrasound • 2-10 million Hz (2-10 MHz)

10. Physics Frequency Inversely proportional to penetration Directly proportional to resolution Probes Transabdominal 3.5, 5, 7 mHz Transvaginal 8-9 mHz

11. Sound waves - Transducer both sends and receives - Reflected by emitting transducer - Image displayed as: 1. Brightness - intensity of echo 2. Time lag - distance

12. Ultrasound and BMI • Heavier patients • Need more penetration (lower mHz) • Get less resolution (lower mHz)

13. What are the types of US – who gets what type of scan?

14. Basic Ultrasound • Examination • Fetal number/presentation/”life” • Placental location • Assessment of AFV • Assessment of gestational age • Survey for “gross” malformations • Evaluation for maternal pelvic masses • Metric examination • Screening

15. Limited Ultrasound • Assessment of AFV, BPP • Guidance for • Amniocentesis • External cephalic version • Confirmation of fetal death • Placental localization (hemorrhage) • Fetal presentation

16. Comprehensive Ultrasound • Indications • Suspicion of anomalous fetus • History • Clinical evaluation • Previous ultrasound • Detailed assessment of fetal anatomy • Color/power doppler • Arterial/venous doppler

17. What type of anomalies is this patient at risk for and how good is ultrasound at finding them?

18. How good is ultrasound at finding anomalies? It depends on: The anomaly Minor anomalies, heart anomalies hardest When we look When apparent, 20 –24 wks optimal for most Who we are looking at Thinner, normal amniotic fluid volume And……. Who is looking.

19. Detection Directly proportional to severity of anomaly - 89% lethal anomalies - 77% requiring NICU admission - 30% minor anomalies

20. Lowest rates Cardiovascular defects Cleft up / palate Microcephalus

21. Basic (76805) Measurements, AFI, placenta Head Heart (not color) Abdomen Comprehensive (76811) Face, profile Extremities Heart Color doppler Extremities Types of Ultrasound – what might be missed?

29. What about antenatal testing?

30. Table 43-1. COMPONENTS AND THEIR SCORES OF THE BIOPHYSICAL PROFILE Variable Score 2 Score 0 Less than 30 sec of fetal breathing movements in 30 min Fetal breathing The presence of at least 30 sec of sustained fetal movements breathing movements in 30 min of observation Fetal movements Three or more gross body movements in 30 min of Two or less gross body movement observation: simultaneous limb and trunk movements in 30 min of observation Fetal tone At least one episode of motion of a limb from position Fetus in position of semi- or of flexion to extension and rapid return to flexionS full-limbextension with no return or slow return to flexion with movement; absence of fetal movement counted as absent tone. Fetal reactivity Two or more fetal heart rate accelerations of least No acceleration or less than 15 beats/min and lasting at least 15 sec and associated two accelerations of fetal with fetal movement in 20 min heart rate in 20 min of observation Qualitative amnionic Pocket of amnionic fluid that measures at least 1 cm Largest pocket of amnionic fluid fluid volume in two perpendicular planes measures< 1 cm in two perpendicular planes From Manning and colleagues (1985), with permission.

32. How well do we estimate fetal weight?

33. Estimated Fetal Weight • Various formulas • All involve the abdominal circumference • Also Femur length, head circumference and/or BPD • Less Accurate in bigger babies (> 4000 grams) • Accuracy + 10 – 15% • Term harder to get measurements • Fetal position AFI

34. Figure 1 (No legend p 524 OB Gyn 1999: 93: 523-6) put in author and year

35. RED CELL ALLOIMMUNIZATIONFrequency of Irregular Antibodies % D Kell Duffy MNS Kidd Lutheran Queenan et al. Obstet Gynecol 1969; 34: 767-70 Geifman-Holtzman et al. Obstet Gynecol 1997; 89: 272-5

36. ACOG recommends antenatal RHIG ACOG recommends antenatal RHIG

37. RED CELL ALLOIMMUNIZATIONRhesus Prophylaxis • 66% of Rhesus cases – antepartum sensitization • 13% of cases – inadvertent omission of RhIG Hughes et al. Brit J Obstet Gynaecol 1994; 101:297-300

38. RED CELL ALLOIMMUNIZATIONNew Onset RhD Sensitization • Follow maternal titers every 2 - 4 weeks until critical value reached (32 at UNC) • Determine paternal genotype for involved antigen

39. RED CELL ALLOIMMUNIZATIONNew Onset RhD Sensitization • Paternal genotype = heterozygous (55%); do amniocentesis for fetal blood typing • Paternal genotype = homozygous (45%) or affected fetus by amniocentesis DNA testing; begin serial amniocenteses for ΔOD450 testing

40. RED CELL ALLOIMMUNIZATIONPrevious RhD Sensitization • History of previous IUFD, intrauterine transfusions or neonatal exchange transfusions • Maternal titers not helpful