Ultrasound is a vital tool in obstetric practice for gestational age estimation, fetal growth monitoring, and anomaly screening, with routine screening performed at 18-20 weeks. Key indications include diagnosing pregnancy, assessing fetal development, and guiding invasive procedures, alongside evaluating complications like threatened abortion and ectopic pregnancy. The document outlines detailed fetal biometry parameters, ultrasound findings for various conditions, and the significance of amniotic fluid and placental assessments in managing pregnancy.

1. 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?
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Generally well accepted for examination with indication
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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
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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
Sonoembryology
Early Fetal Development
(Transvaginal Sonography; TVS)
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3-4 weeks (after LMP): Endometrial thickenings
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4-5 weeks: Gestational sac
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5-6 weeks: Yolk sac, double decidual sac sign (DDS)
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6-7 weeks: Embryo with heart beat
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7-8 weeks: Embryo movement, Rhombencephalon, Amnion
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8-9 weeks: Physiologic omphalocele, limbs, choroid plexus, spinal line
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TVS demonstrates earlier than transabdomen (TAS) ~ 1-2 weeks

2. 4-5 weeks
5 weeks
Endometrial thickening, no obvious sac
Gestational ring, white echogenic rim

3. Double decidual sac sign
5 weeks
Usually seen at 5-6 weeks
Note yok sac
6 weeks
7-8 weeks
Note: yok sac and early embryo
Rapid growth of embryo and amnion
8 weeks
10 weeks
Fetal body compartments : head trunk limbs
Facial structures becoming seen
Double Decidual Sac Sign (DDS)
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Strongly suggestive of intrauterine pregnancy

4. •
Outer ring : Decidual vera
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Middle sonolucency : Endometrial cavity
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Inner ring : Decidual capsularis
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Typically seen : 5-8 weeks
Double decidual sac sign
5 weeks
Usually seen at 5-6 weeks
Note yok sac
Mean Sac Diameter (MSD)
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MSD : Average gestational sac diameter = width + depth + length / 3
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When MSD > 25 mm. GA (days) = MSD + 30
MSD closely related to early GA

5. Mean sac diameter
Mean sac diameter
Longitudinal diameter
Transverse diameter and depth
Yolk Sac
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Round, sonolucent with white border
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Average 5 mm (3-8 mm)
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Seen at 6-12 weeks peak 8-10 weeks
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Nearly all seen when MSD > 8 mm.
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Yolk sac > 10 mm. related to poor prognosis
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Must be seen if MSD > 20 mm. by TAS or > 13 mm by TVS
Fetal Echo
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Crown-Rump Length (CRL): the most accurate parameter for GA estimation (+ 3-7 days)
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Short CRL related to high abortion rate and aneuploidy
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Head, trunk, limbs can be identified from 8 weeks
Useful only in the first trimester
Amnion
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Seen from 7-8 weeks (TVS)
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Fast growing, finally embryo is in the sac
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Yolk sac is outside
Beginning with double bleb sign (yolk sac and amnionic sac)

6. Yolk sac
Yolk sac
Yolk sac adjacent to fetal echo in early gestation
Note: yolk sac separated from amnion
Physiologic Omphalocele
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Midgut herniation 8-12 weeks
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Only bowel (no liver) in the proximal umbilical cord
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Not seen if CRL > 44 mm
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Size 4-7 mm.
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Should be considered abnormal if > 7 mm, or seen after 12 weeks

7. Physiologic omphalocele
Physiologic omphalocele
Prominent at 8-9 weeks
At 8 weeks
Physiologic omphalocele
Physiologic omphalocele in abortus
Nuchal Translucency (NT)
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NT : small fluid collection beneath skin at back of the fetal neck
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Measured at CRL 35-80 mm (10-14 week)
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Measured on midsagittal scan (plane for CRL)
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The best sonomarker for screening Down syndrome
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Abnormal if > 95th percentile (> 2.5-3.0 mm)
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Thickened NT increased of aneuploidy, anomaly especially cardiac defect

8. Nuchal translucency
Nuchal translucency
Normal measured at 11 weeks
Thickened nuchal translucency
Nuchal translucency
Nuchal translucency
Thickened nuchal translucency at 14 weeks
Thickened nuchal translucency after abortion
Early Pregnancy Complications
Threatened Abortion
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Ultrasound examinations see if viable or nonviable pregnancy
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Viable fetus with normal heart beat : very good prognosis
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Nonviable:

9. o
blighted ovum
o
missed / incomplete abortion
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fetal death
o
ectopic pregnancy
o
molar pregnancy
Threatened abortion
Threatened abortion
Normal fetus at 11 weeks
Placental hematoma in case of blighted ovum

10. Early embryo death
Embryo size and sac disproportion
Blighted Ovum (Empty sac)
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Intrauterine pregnancy without embryo
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Diagnosed when
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MSD > 25 mm (TAS) or > 20 mm (TVS) with no embryo seen
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MSD > 20 mm (TAS) or >17 mm (TVS) with no yolk sac & embryo seen
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DDx :
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Early normal pregancy
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Pseudosac in ectopic pregnancy
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Blood or fluid collection

11. Blighted ovum
Blighted ovum
Gestational sac without embryo, subcorion hematoma
Gestational sac without embryo,
Blighted ovum
Aborted sac: placenta and sac without embryo
Ectopic Pregnancy
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Clinically suspected with stable vital sign : ultrasound
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Ultrasound results:
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Definite IUP : exclude ectopic pregnancy
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Definite EUP : extrauterine gestational sac
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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)
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Inconclusive : empty uterus without other abnormal finding (May need doubling time for beta-hCG)

12. Ectopic pregnancy
Ectopic pregnancy
Floating dilated fallopian tube in free fluid
Gestational sac with embryo and yolk sac in the tube
Ectopic pregnancy
Adnexal omplex mass of blood clot and concepitus
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Molar Pregnancy
o
o
Ultrasound findings:
o
Snow storm pattern or
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Numerous small cystic echo or
No fetus

13. o
Placental-like echo
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May show complex area of blood clot
Molar pregnancy
Molar pregnancy
Numerous small cystic space in uterine cavity mass
Snow storm appearance
Molar pregnancy
The opened uterine specimen after hysterectomy
Fetal Biometry
Mean Sac Diameter (MSD)
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MSD : Average gestational sac diameter = width + depth + length / 3

14. •
MSD closely related to early GA
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When MSD > 25 mm. GA (days) = MSD + 30
Mean sac diameter
Mean sac diameter
Longitudinal diameter
Transverse diameter and depth
Crown-Rump Length (CRL)
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The most accurate parameter for GA (+ 3-7 days)
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Limitation: Appropriate only in first trimester
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Technique:
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Mid-sagittal scan (note fetal nose, spine, crown and rump)
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Measurement from the topmost of head to rump end
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Precaution: best done in neutral position, not include yolk sac or limbs
Most accurate during 6.5-10 weeks

15. Crown-rump length
Crown-rump length
8 weeks
9 weeks
Crown-rump length
12 weeks
Biparietal Diameter (BPD)
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The best parameter during 2nd trimester (+ 7-11 days during 14-26 weeks)
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Ovoid and symmetry
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Thalamus
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Midline echo / third ventricle
Technique: the distance from outer-to-inner skull table in the plane visualized of

16. •
Cavum septum pellucidum
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Limitation: less reliable in case of
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Cephalic index (CI: BPD/OFD x 100) < 75% (dolichocephaly) or > 85% (brachycephaly) (normal CI 85%;
75-85%)
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Irregular skull shape or hydrocephalus
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Varied in 3rd trimester (+ 2-3 wks)
Head Circumference (HC)
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Measurement on the same plane as BPD
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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

17. Biparietal diameter
Biparietal diameter
Standard plane for BPD measurement
Standard plane for BPD measurement
Dolichocephaly
Brachycephaly
BPD not proper for gestatational age calculation
BPD not proper for gestatational age calculation
Abdominal Circumferece (AC)
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Most varied among the standard parameter
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Less accurate for GA estimation
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Best parameter for fetal growth evaluation or estimate fetal weight
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Plane for AC:
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as round as possible
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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)
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stomach
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Measurement: perimeter around fetal skin
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Limitation: not accurate for GA, not round due to pressure effect

18. Abdominal circumference
Abdominal circumference
Standard plane for abdominal circumference
Standard plane for abdominal circumference
measurement
measurement
Abdominal circumference
Standard plane for abdominal circumference
measurement
Femur Length (FL)
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The accuracy for GA similar to BPD, may be slightly less accurate and more accurate than BPD in 2nd
and 3rd trimester respectively

19. •
Plane: the longest plane and straight with least curve as possible
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Measurement between the both end, not include epiphysis
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Precaution: FL among Thai is shorter than that of western pregnanc
Femur lenght
Femur lenght
Standard plane for femur length measurement
Standard plane for femur length measurement
Femur lenght
Standard plane for femur length measurement

20. Fetal Growth Restriction (FGR)
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AC : most commonly used for diagnosis
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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
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FL/AC ratio : (date-independent) constant after 20 wk (normal ratio ~22+2 abnormal if > 24), unreliable
for symmetrical FGR
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Umbilical artery Doppler waveforms: high resistance or absent end-diastole for true FGR but normal for
constitutional small fetus
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Oligohydramnios is common among FGR
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Estimate fetal weight (< 10th percentile)
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Grade 3 placenta before 36 week
Fetal Growth Restriction (FGR)
Fetal Growth Restriction (FGR)
FGR due to twin-to-twin transfusion syndrome
FGR due to twin-to-twin transfusion syndrome
Placenta & Amniotic Fluid
Amniotic Fluid
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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
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Polyhydramnios (AFI > 95th centile or > 20-25) : commonly related to maternal DM, anomaly i.e.
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esophageal atresia
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neural tube defects
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aneuploidy etc.

21. Amniotic fluid index
Polyhydramnios
Four quadrant deepest verical pocket measurement
Polyhydramnios due to fetal anencephaly
Placental grading
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0 : no calcifications
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1 : scattered calcifications
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2 : basal calcifications
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3 : basal and septal calcification; outline the cotyledons; commonly seen in postterm, FGR, PIH
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Extensive calcification < 36 wk related to FGR

22. Placental Grading
Placental Grade 0

23. Placental Grade 1
Placental Grade2
Placental Grade 3
Placenta Previa
Marginal previa : adjacent to the internal os
o
Partial previa: placenta covers a portion of internal os (indistinguishable from marginal previa in
o
prenatal practice)
o
Total previa: placenta covers the os
o
Low-lying placenta: nearly the os, not true previa and vaginal deliver is possible
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Ultrasound: should be done with an empty bladder because the cervix is spuriously long by full
bladder leading to false previa
o
Most placenta previa diagnosed in the 2nd trimester is away from the os at term
o
The cervix could be visualized using TAS, TVS or transperineal approach

24. Placenta previa totalis
Placenta previa totalis
Standard plane for BPD measurement
Standard plane for BPD measurement
False placenta previa totalis
False placenta previa totalis
Full bladder compress lower segment, simulating placenta
previa totalis
The same case (after voiding)
Placental Abruption
o
o
cystic, complex, or hypoechoic areas may be seen between placenta and uterine wall
o
reveal type may be not diagnosed
Placental thickening

25. retro placental hematoma may be isoechoic like placenta
o
Placental abruption
Placental abruption
Placental abruption
Placental abruption
Fetal anomaly
Fetal Hydrops
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Fluid accumulation : subcutaneous edema, ascites, pleural effusion, pericardial effusion,
placentomegaly
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Most due to Hb Bart’s disease (1 : 1000 birth in northern Thailand), usually not related to other anomaly

26. •
Other causes
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Rh isoimmunization
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Fetal anomaly: cystic hygroma, cardiac anomaly, supraventricular tachycardia
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Aneuploidy: 45XO, Down syndrome
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Infections: parvovirus B 19, syphilis
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Miscellaneous: chorioangioma, twin-twin transfusion syndrome etc.
Sonographic Findings of Hb Bart’s disease
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cardiomegaly (increased cardio-thoracic ratio from midpregnancy) (The earliest sign)
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Placentomegaly
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Ascites
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Pleural or pericardial effusion
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Subcutaneous edema (late sign)
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Oligohydramnios (in late pregnancy) (unlike other causes which commonly related to polyhydramnios)

27. Hydrops fetalis
Hydropic placenta
Hydropic fetalis due to Hb Bart's diisease
Hydropic fetalis due to Hb Bart's diisease
Cardiomegaly
Ascites
Markedly enlarged heart in fetal Hb Bart's diisease
Ascites in fetal Hb Bart's diisease
Subcutaneous edema
Scalp edema in fetal Hb Bart's diisease
Anencephaly

28. •
The most common NTDs (1: 1000 births)
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Ultrasound findings
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Absent skull
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Prominent orbit
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Often related to polyhydramnios
Anencephaly
Anencephaly
Base of skull contact with uterine wall / polyhydramnios
No skull above the orbits : Spectacle sign
Anencephaly
Postnatal appearance of a term anencephalic fetus

29. Ventriculomegaly
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Enlargement of cerebral ventricles or with increased pressure (hydrocephalus)
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Most cases of marked ventriculomegaly caused by obstruction of aqueduct of Sylvious
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Ventriculomegaly (> 10 mm)
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Dilated 3rd ventricle (> 3 mm)
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Dangling choroid plexus sign
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Thin cerebral mantle
Hydrocephalus
Hydrocephalus
Markedly enalarged lateral ventricles
Autopsy : markedly enalarged lateral ventricles
Cystic Hygroma
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Lymph collections due to obstruction, especially jugular lymph sac
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Cyst at the posterolateral neck, septate or nonseptate
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Lethal if hydrops occurs, but simple cyst may regress and disappear
Commonly associated with 45XO (70%), and trisomy 21, 18

30. Cystic hygroma
Cystic hygroma
Septate cyst at the back of fetal neck
Postnatal finding
Omphalocele
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A protrusion of bowel / liver through abdominal wall at the umbilicus
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The protrusion covered by a membrane
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50% associated with other anomalies, especially cardiac defects
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If containing bowel, 80% associated with abnormal chromosomes
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Liver-containing omphalocele: 20% associated with abnormal chromosomes

31. Omphalocele
Omphalocele
Protruding mass containg liver with membrane covering Note: extra-abdominal mass with covering membrane
Gastroschisis
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A protrusion of bowel (rarely other visceral organ) through a defect of the abdominal wall, typically to the
right of the cord insertion
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No membrane covers the mass
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Not related to chromosome abnormalities or other anomalies other than GI
Gastrochisis
Gastrochisis
Free floating bowels in amniotic fluid
Postnatal appearance: no covering membrane
Hydronephrosis
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> 75% related to renal abnormalities
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Ureteropelvic junction (UPJ) obstruction is the most common cause: dilated renal pelvis (> 10 mm) and
calyces, often bilateral
•
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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

32. Hydronephrosis
Hydronephrosis
Dilated renal pelvis and calyces
Dilated renal pelvis and calyces