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
|
Trisomy 18, 13, triploidy
|
Face:
|
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:
|
Trisomy 13, 18, 21
Triploidy |
Hands/feet:
|
Trisomy 18
Trisomy 13
Trisomy 21 |
GI system:
|
Trisomy 13, 18
Trisomy 21 |
General
|
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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