Basic Obstetric Ultrasound

Why Ultrasound?

Appropriate use is helpful in obstetric practice, especially in gestational age estimation, fetal growth monitoring, obstetric hemorrhage, and anomaly screening

Indicated or Routine Ultrasound?

• Generally well accepted for examination with indication
• Routine screening at 18-20 weeks, recent more common practice, better or earlier diagnosis of GA, twins or anomaly but significant increase in cost and workload
• The policy must be considered for cost-effectiveness and cost-benefit

Common Indications

1. Diagnosis: pregnancy, number of fetuses, fetal life
2. Size inconsistent with date: multiple pregnancy, oligo-, polyhydramnios, hydrocephalus, fetal growth restriction
3. Estimate gestational age
4. Growth monitoring
5. Bleeding: abortion, placenta previa, placental abrutpion
6. Amniotic fluid evaluation
7. Pathology in the pelvis
8. Anomaly screening: Routine at 18-20 wk or pregnancy at risk (maternal DM, familial history,advanced maternal age)
9. Guidance for invasive procedures, i.e. amniocentesis, cordocentesis

{tab=Sonoembryology}

Sonoembryology

Early Fetal Development

(Transvaginal Sonography; TVS)

• 3-4 weeks (after LMP): Endometrial thickenings
• 4-5 weeks: Gestational sac
• 5-6 weeks: Yolk sac, double decidual sac sign (DDS)
• 6-7 weeks: Embryo with heart beat
• 7-8 weeks: Embryo movement, Rhombencephalon, Amnion
• 8-9 weeks: Physiologic omphalocele, limbs, choroid plexus, spinal line
• TVS demonstrates earlier than transabdomen (TAS) ~ 1-2 weeks

Double Decidual Sac Sign (DDS)

• • Strongly suggestive of intrauterine pregnancy
  • Outer ring : Decidual vera
  • Middle sonolucency : Endometrial cavity
  • Inner ring : Decidual capsularis
  • Typically seen : 5-8 weeks

Mean Sac Diameter (MSD)

• • MSD : Average gestational sac diameter = width + depth + length / 3
  • MSD closely related to early GA
  • When MSD > 25 mm. GA (days) = MSD + 30

Yolk Sac

• • Round, sonolucent with white border
  • Average 5 mm (3-8 mm)
  • Seen at 6-12 weeks peak 8-10 weeks
  • Nearly all seen when MSD > 8 mm.
  • Yolk sac > 10 mm. related to poor prognosis
  • Must be seen if MSD > 20 mm. by TAS or > 13 mm by TVS

Fetal Echo

• • Crown-Rump Length (CRL): the most accurate parameter for GA estimation (+ 3-7 days)
  • Useful only in the first trimester
  • Short CRL related to high abortion rate and aneuploidy
  • Head, trunk, limbs can be identified from 8 weeks

Amnion

• • Seen from 7-8 weeks (TVS)
  • Beginning with double bleb sign (yolk sac and amnionic sac)
  • Fast growing, finally embryo is in the sac
  • Yolk sac is outside

Physiologic Omphalocele

• • Midgut herniation 8-12 weeks
  • Only bowel (no liver) in the proximal umbilical cord
  • Not seen if CRL > 44 mm
  • Size 4-7 mm.
  • Should be considered abnormal if > 7 mm, or seen after 12 weeks

Nuchal Translucency (NT)

• • NT : small fluid collection beneath skin at back of the fetal neck
  • Measured at CRL 35-80 mm (10-14 week)
  • Measured on midsagittal scan (plane for CRL)
  • The best sonomarker for screening Down syndrome
  • Abnormal if > 95th percentile (> 2.5-3.0 mm)
  • Thickened NT increased of aneuploidy, anomaly especially cardiac defect

Early Pregnancy Complications

Threatened Abortion

• • Ultrasound examinations see if viable or nonviable pregnancy
  • Viable fetus with normal heart beat : very good prognosis
  • Nonviable:
    • blighted ovum
    • missed / incomplete abortion
    • fetal death
    • ectopic pregnancy
    • molar pregnancy

Blighted Ovum (Empty sac)

• • Intrauterine pregnancy without embryo
  • Diagnosed when
  • MSD > 25 mm (TAS) or > 20 mm (TVS) with no embryo seen
  • MSD > 20 mm (TAS) or >17 mm (TVS) with no yolk sac & embryo seen
  • DDx :
    • Early normal pregancy
    • Pseudosac in ectopic pregnancy
    • Blood or fluid collection

Ectopic Pregnancy

• • Clinically suspected with stable vital sign : ultrasound
  • Ultrasound results:
  • Definite IUP : exclude ectopic pregnancy
  • Definite EUP : extrauterine gestational sac
  • Highly suggestive of EUP : empty uterus with complex mass (separate from ovaries), echogenic fluid, dilated tube (May treat EUP or laparoscopic diagnsois in some cases)
  • Inconclusive : empty uterus without other abnormal finding (May need doubling time for beta-hCG)

Molar Pregnancy

• • Ultrasound findings:
  • No fetus
  • Snow storm pattern or
  • Numerous small cystic echo or
  • Placental-like echo
  • May show complex area of blood clot

Fetal Biometry

Mean Sac Diameter (MSD)

• • MSD : Average gestational sac diameter = width + depth + length / 3
  • MSD closely related to early GA
  • When MSD > 25 mm. GA (days) = MSD + 30

Crown-Rump Length (CRL)

• • The most accurate parameter for GA (+ 3-7 days)
  • Most accurate during 6.5-10 weeks
  • Limitation: Appropriate only in first trimester
  • Technique:
  • Mid-sagittal scan (note fetal nose, spine, crown and rump)
  • Measurement from the topmost of head to rump end
  • Precaution: best done in neutral position, not include yolk sac or limbs

Biparietal Diameter (BPD)

• • The best parameter during 2nd trimester (+ 7-11 days during 14-26 weeks)
  • Technique: the distance from outer-to-inner skull table in the plane visualized of
  • Ovoid and symmetry
  • Thalamus
  • Midline echo / third ventricle
  • Cavum septum pellucidum
  • Limitation: less reliable in case of
  • Cephalic index (CI: BPD/OFD x 100) < 75% (dolichocephaly) or > 85% (brachycephaly) (normal CI 85%; 75-85%)
  • Irregular skull shape or hydrocephalus
  • Varied in 3rd trimester (+ 2-3 wks)

Head Circumference (HC)

• • Measurement on the same plane as BPD
  • The accuracy similar to BPD (+ 1 wk before 20 wk and + 2-3 wk in the 3rd trimester)
  • Theoretically better than BPD, but practically less accurate due to poor imaging of anterior and posterior of the skull secondary to acoustic shadow

Abdominal Circumferece (AC)

• • Most varied among the standard parameter
  • Less accurate for GA estimation
  • Best parameter for fetal growth evaluation or estimate fetal weight
  • Plane for AC:
  • as round as possible
  • umbilical vein (middle-third) running to portal sinus in the liver (Note: if umbilical vein seen closely to anterior wall the plane is too low or oblique)
  • stomach
  • Measurement: perimeter around fetal skin
  • Limitation: not accurate for GA, not round due to pressure effect

Femur Length (FL)

• • The accuracy for GA similar to BPD, may be slightly less accurate and more accurate than BPD in 2nd and 3rd trimester respectively
  • Plane: the longest plane and straight with least curve as possible
  • Measurement between the both end, not include epiphysis
  • Precaution: FL among Thai is shorter than that of western pregnanc

Fetal Growth Restriction (FGR)

• • AC : most commonly used for diagnosis
  • HC/AC ratio : increased in FGR ( the ratio is date dependent ; decreasing with GA, > 1 before 32 week, ~ 1 during 32-36 wk, > 1 after 36 wk) unreliable for symmetrical FGR
  • FL/AC ratio : (date-independent) constant after 20 wk (normal ratio ~22+2 abnormal if > 24), unreliable for symmetrical FGR
  • Umbilical artery Doppler waveforms: high resistance or absent end-diastole for true FGR but normal for constitutional small fetus
  • Oligohydramnios is common among FGR
  • Estimate fetal weight (< 10th percentile)
  • Grade 3 placenta before 36 week

Placenta & Amniotic Fluid

Amniotic Fluid

• • Amniotic fluid index (AFI): Sum of the four deepest depth of AF four quadrant
  • Oligohydramnios (AFI < 5) : commonly associated with FGR, rupture of membranes, and anomaly i.e. renal agenesis, polycystic kidney
  • Polyhydramnios (AFI > 95th centile or > 20-25) : commonly related to maternal DM, anomaly i.e.
  • esophageal atresia
  • neural tube defects
  • aneuploidy etc.

Placental grading

• • 0 : no calcifications
  • 1 : scattered calcifications
  • 2 : basal calcifications
  • 3 : basal and septal calcification; outline the cotyledons; commonly seen in postterm, FGR, PIH
  • Extensive calcification < 36 wk related to FGR

Placenta Previa

• • Marginal previa : adjacent to the internal os
  • Partial previa: placenta covers a portion of internal os (indistinguishable from marginal previa in prenatal practice)
  • Total previa: placenta covers the os
  • Low-lying placenta: nearly the os, not true previa and vaginal deliver is possible
  • Ultrasound: should be done with an empty bladder because the cervix is spuriously long by full bladder leading to false previa
  • Most placenta previa diagnosed in the 2nd trimester is away from the os at term
  • The cervix could be visualized using TAS, TVS or transperineal approach

Placental Abruption

• • cystic, complex, or hypoechoic areas may be seen between placenta and uterine wall
  • Placental thickening
  • reveal type may be not diagnosed
  • retro placental hematoma may be isoechoic like placenta

Fetal anomaly

Fetal Hydrops

• • Fluid accumulation : subcutaneous edema, ascites, pleural effusion, pericardial effusion, placentomegaly
  • Most due to Hb Bart’s disease (1 : 1000 birth in northern Thailand), usually not related to other anomaly
  • Other causes
  • Rh isoimmunization
  • Fetal anomaly: cystic hygroma, cardiac anomaly, supraventricular tachycardia
  • Aneuploidy: 45XO, Down syndrome
  • Infections: parvovirus B 19, syphilis
  • Miscellaneous: chorioangioma, twin-twin transfusion syndrome etc.

Sonographic Findings of Hb Bart’s disease

• • cardiomegaly (increased cardio-thoracic ratio from midpregnancy) (The earliest sign)
  • Placentomegaly
  • Ascites
  • Pleural or pericardial effusion
  • Subcutaneous edema (late sign)
  • Oligohydramnios (in late pregnancy) (unlike other causes which commonly related to polyhydramnios)

Anencephaly

• • The most common NTDs (1: 1000 births)
  • Ultrasound findings
  • Absent skull
  • Prominent orbit
  • Often related to polyhydramnios

Ventriculomegaly

• • Enlargement of cerebral ventricles or with increased pressure (hydrocephalus)
  • Most cases of marked ventriculomegaly caused by obstruction of aqueduct of Sylvious
  • Ventriculomegaly (> 10 mm)
  • Dilated 3rd ventricle (> 3 mm)
  • Dangling choroid plexus sign
  • Thin cerebral mantle

Cystic Hygroma

• • Lymph collections due to obstruction, especially jugular lymph sac
  • Commonly associated with 45XO (70%), and trisomy 21, 18
  • Cyst at the posterolateral neck, septate or nonseptate
  • Lethal if hydrops occurs, but simple cyst may regress and disappear

Omphalocele

• • A protrusion of bowel / liver through abdominal wall at the umbilicus
  • The protrusion covered by a membrane
  • 50% associated with other anomalies, especially cardiac defects
  • If containing bowel, 80% associated with abnormal chromosomes
  • Liver-containing omphalocele: 20% associated with abnormal chromosomes

Gastroschisis

• • A protrusion of bowel (rarely other visceral organ) through a defect of the abdominal wall, typically to the right of the cord insertion
  • No membrane covers the mass
  • Not related to chromosome abnormalities or other anomalies other than GI

Hydronephrosis

• • > 75% related to renal abnormalities
  • Ureteropelvic junction (UPJ) obstruction is the most common cause: dilated renal pelvis (> 10 mm) and calyces, often bilateral
  • Thin renal parenchyma suggestive of poor renal function
  • Renal pelvic dilation < 10 mm is often a normal variant but needs follow up and slightly increased risk of Down syndrome