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ROLE OF USG IN OBSTETRICS

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ROLE OF USG IN OBSTETRICS

INDICATIONS:

Indication of First Trimester Ultrasonography :

  • Intrauterine pregnancy
  • Suspected ectopic pregnancy
  • Vaginal bleeding (in all trimesters)
  • Fetal anomalies (anencephaly 10 weeks)
  • Suspected molar pregnancy
  • Gestational age
  • Multiple pregnancy (chorionicity)
  • To confirm cardiac activity(6weeks)
  • Screening of aneuploidy
  • Evaluation of pelvic/adnexal masses (all trimesters)

Indication of Second and Third Trimester USG

  • Gestational age
  • Cervical insufficiency
  • Suspected multiple pregnancy
  • Liquor volume (poly/oligohydramnios)
  • Placenta previa/abruption
  • Suspected uterine malformation
  • Cases with PROM
  • Fetal growth (FGR)
  • Fetal presentation (breech,face)
  • Fetal well-being (BPP) assessment
  • Screening fetal anomalies
  • As an adjunct to: amniocentesis,CVS, cordocentesis,fetal therapy, ECV
  • Uterine size: Either > dates or < dates
FIRST TRIMESTER ULTRASONOGRAPHY:

Intrauterine gestational Sac(GS) should be seen:

  • Transvaginal Sonography (TVS) when β-hCG level is 1,000–1,200 mIU/mL
  • Tabdominal Sonography(TAS)  when β-hCG is 6,000 mIU/mL

OBSERVATIONS:

  • True Gestational Sac (GS):Eccentric in endometrium of fundus or body of the uterus.
  • Double decidua sign:Interface between the decidua and the chorion
  • Yolk sac or fetal pole in GS: confirms pregnancy.
  • GS size increase by 1.1 mm/day

Fetal anatomy and viability:

Mean Sac Diameter (MSD) Findings
5–8 mm Yolk sac
12 mm Embryo
15–18 mm Cardiac activity
Embryo CRL > 4 mm Cardiac activity

Gestational age and Embryonic Structures:

Menstrual Age  Fetal Structures
4 weeks Choriodecidual thickness,
chorionic sac
5 weeks Gestation sac, embryo yolk sac
6 weeks Fetal pole, cardiac activity
7 weeks Lower limb buds, midgut
herniation (physiological)
8 weeks Upper limb buds, stomach
9 weeks Spine, choroid plexus

Ultrasound markers for fetal anomalies:

  • Nuchal translucency:Increased fetal nuchal skin thickness>3 mm:Chromosomal anomalies (trisomy 21, 18, 13, triploidy and Turner’s syndrome).
  • Multiple pregnancy:2 gestational sacs indicates twin birth in 52–63% of cases
  • Ectopic pregnancy:Echogenic fluid in the pouch of Douglas (blood) 
  • Absence of gestational sac in uterus  & Adenexal mass
  • Color Doppler:Identify Echogenic ring(ring-of-fire) of an ectopic gestational sac
MIDTRIMESTER ULTRASONOGRAPHY:
Fetal growth:
  • Normal:10th-90th percentiles
  • Biparietal diameter (BPD):(12 week )Outer skull edge of proximal skull to inner edge on distal skull
  • Head circumference (HC)
  • Abdominal circumference (AC):Measured at junction of left and right portal veins & umbilical vein
  • Femur length (FL):Measured when beam from the transducer is perpendicular to the shaft
  • Brith weight(BW)

Hadlock formula :

  •  log(10)BW = 1.335 – 0.0034(AC)(FL) + 0.0316(BPD) + 0.0457(AC) + 0.1623(FL)
Gestational age assessment:
  • Optimum time for most accurate assessment :14 -20 weeks by Crown rump length
  • Transcerebellar diameter (TCD):14 weeks- 28 weeks
  • Dating ultrasound done before 22 weeks should be used in preference to menstrual dates
Fetal gender identification:
  • Detection of testes within the scrotum( 3rd trimester)
Placenta and umbilical cord:
  • Normal thickness: 30mm
  • Abnormal thickness: >45mm
  • Relationship of placenta to the internal cervical os is important to define low lying placenta and placenta previa
  • Transperineal or transvaginal imaging
Placenta of multifetal pregnancy (Chorionicity):
  • Diamnioticdichorionic(DiDi) placenta:Dizygotic twins & 20–30% of monozygotic twins
    • U/S:twin peak sign
    • DiDi twins:thickness of dividing membrane>2 mm
3D Power Doppler:
  • Shows vascular lakes, with turbulent flow with peak systolic velocity >15 cm/s.
Umbilical cord
  • Single umbilical artery (SUA):Higher rate of fetal anomalies (30–70%).
  • Umbilical cord insertion: Velamentous insertion,and vasa previa diagnosed with Doppler ultrasound
Assessment of amniotic fluid:
  • AFI >25 cm constitutes polyhydramnios
Doppler:
  • Direction as well as velocity of blood flow can be measured by Doppler ultrasound
Routine ultrasonography:
  • 18–22 weeks
  • Reduces the incidence of post-term pregnancy (39%) and rates of induction of labor for post-term pregnancy.
Ultrasound Markers of Chromosomal Abnormalities (13-19 weeks):
Observation Chromosomal
Abnormality
Head
  • Choroid plexus cyst
  • Strawberry skull
  • Hydrocephalus
  • Holoprosencephaly
  Trisomy 18, 13, triploidy

 

 Face:
  • Cleft lip/palate
  • Micrognathia
  • Low set ears
  Trisomy 13, 18,

Meckel-Gruber syndrome

Triploidy

 Nuchal translucency>3 mm  Trisomy 21, 18, 13,Turner syndrome
 Cardiac defects  Trisomy 13, 18, 21
 Renal anomalies:
  • Horseshoe kidney
  • Bilateral dilatation of renal pelvis
  • Cystic dysplasia
 Trisomy 13, 18, 21

Triploidy

 Hands/feet:
  • Flexed overlapping fingers
  • Rockerbottom/club foot
  • Polydactyly
  • Wide gap between 1st and 2nd toes
  • Clinodactyly
  • Short femur
 Trisomy 18

Trisomy 13

 

Trisomy 21

 GI system:
  • Echogenic bowel
  • Omphalocele
  • Esophageal atresia
  • Duodenal atresia
 Trisomy 13, 18

Trisomy 21

 General
  • Early IUGR
  • Hydrops
  Trisomy 13, 18, 21

Triploidy, 45XO

THIRD TRIMESTER ULTRASONOGRAPHY:
  • All the information of second trimester sonography can be obtained in third trimester.
  • Detailed anatomic survey:Achondroplastic dwarfism is diagnosed in this trimester.
  • Estimated fetal weight (EFW) is determined from the average of three readings for:
    • FL
    • AC is most important.
    • BPD.  
  • Growth profile
    • In asymmetric IUGR, the HC is maintained but the AC falls off around 30 weeks, HC:AC ratio is therefore elevated. 
    • In symmetric IUGR, both the HC and AC are affected early,HC:AC ratio remains normal.

Exam Important

  • Transvaginal USG can detect fetal cardiac activity in 6 weeks 
  • Earliest detection of pregnancy by ultrasound is by Gestation sac
  • Study of fetal parts in first trimester with least radiation hazard is USG
  • Investigation of choice in diabetic mother with a doubtful abnormal fetus USG
  • Earliest fetal anomaly to be detected by USG is Anencephaly
  • Accurate diagnosis of anencephaly on ultrasound can be done at 10 weeks of gestation
  • Ultrasound is done in 1st trimester for Multiple pregnancy, Fetal anomalies & To estimate gestational age
  • Congenital anomalies that can be identified by USG  Hydrops fetalis, Esophageal atresia & Anencephaly
  • Best time for diagnosing fetal abnormalities by USG 13-19 weeks of pregnancy
  • Embryonic structure, identified earliest on USG, for confirmation of pregnancy is Gestation sac
  • First trimester USG finding in Down syndrome is Nuchal translucency
  • Best parameter by USG to assess fetal maturity Biparietel diameter at 12 weeks
  • Best time to do USG in pregnancy, if it is to be done once in entire pregnancy, would be18-22 weeks
  • Most accurate assessment of gestational age by USG is done by Crown rump length
  • Ectopic pregnacny, characteristic finding in USG is Absence of gestational sac in uterus 
  • Adenexal mass,Hyprechoic ring & Pseudo sac are signs /features of ectopic pregnancy on USG 
  • USG can detect gestation sac earliest at 5-6 weeks of gestation
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