Sonographic Determination of MA, Multifetal Gestations

Chapter 6. Sonographic determination of MA


  • 임신 1 기

G-sac (chorionic sac) ? bright echogenic ring notd near the endometrial cavity


6wks (TA), 5wks (TV)


MSD (average internal diameter of the G-sac) = (AP + transverse + longitudinal)/3

Obtained from the interface of the chorionic vili and chorionic fluid

Until an MSD of 25mm is reached, GA = 30+MSD


DDx from G-sac & pseudoG-sac

  • INTRADECIDUAL SIGN: imbedded in the deciduas, deviates the endometrial cavity reflection


Embryo 가 보이면 CRL 을 잰다 . 측정이 빠를수록 더 정확하다 .

Cardiac activity: 6wks

MSLL(maximal straight-line length), true CRL

Optimal time of MA from CRL: 6~9wks

Rule of thumb: chorionic sac with no yolk sac or embryo: 5 M wks

chorionic sac with yolk sac but o embryo: 5.5 M wks

chorionic sac with a tiny embryo (<5mm) adjacent to the yolk sac: 6 M wks

10 ~13wks : CRL length

earlier measurement supersedes a later measurement


  • 임신 2 기 ~ 3 기


BPD: any plane of section that transverse the 3 rd ventricle and thalami

Accurate measurement cqan be abtained through infinite number

Calvaria smooth and symmetrically


Accurate predictor of the menstrual age before 20weeks

In certain circumstances (ROM, breech, multiple gestations), shape chane in the fetal head

FOD: fronto-occipital diameter(FOD)

Cephalic index = BPD/FOD x 100

Cephalic index > 1SD, <1SD (<74, >83) ? significant ALTERATION IN BPD e HC can be used


HC: correct plane of section parallels the base of the skull

e CSP must be visible in the ant portion of the brain,

tentoral hiatus must be visible in the post portion of the brain


entire perimeter of the calvaria need not to be demonstrated

large fetus e error will be great

fit to calvarial edge

FL: ossified portion of the daiphysis and metaphysic are measured ( 연골 제외 )

1) Proper alignment of the transducer to the long axis of the femur: both the femoral head or

greater trochanter and the femoral condyle are simulteously in the plane of section

        • correct end points: the cartilages ends, at the junction of the bone with the cartilage
        • distal femoral point is not part of the bony metapysis

AC: measured in a location that estimates liver size

  • liver is the largest organ in the fetal torso,
  • its size reflects aberrations of growth, bony growth restriction and macrosomia
  • transverse diameter of the liver is the greatest
  • the position where the Rt & Lt portal v. are continuous with one another
  • the appearance of the lower ribs is symmetric
  • the shortest length of the umbilical segment of LPV
  • eclipse is fit to skin edge
  • “round” covers a multiude of sins
  • excessive pressure with the transducer should be avoided because it distorts the shape of the abdomen


late pregnancy

after 32wk, visualize distal femoral epiphysis

after 35wks, proximal tibial epiphysis ossification center

the size of the distal osscification center: correlates with

late GA

apprear osscification center appear earlier in female


Chapter 7. Multifetal gestations


multiple pregnancy: 1.5% in US ? infertility treatment & aging maternal population

Twin birth: 1/90 pregnancy, 10% of all perinatal morbidity & mortality

The perinatal mortality rate for twin: 4~6 times > singletons



  • Ultrasound determination of chorionicity & amnionicity


  • perinatal mortality rate for monozygote twin: 3 times higher than for dizygotic twin


A. 1st trimester determination of chorionicity & amnionicity


① chorionicity = the number of G-sac (6 GA~10 GA)

  • singleton: 5 GA, multiple gestation: 6GA 에 G-sac
  • early in the development, amniotic membrane is closely applied to the forming embryo,


and the fluid-filled G-sac is predominantly chorionic fluid and representative of the chorionic cavity

10GA, amniotic cavity enlarge e obliterate chorionic cavity

② amnionicity

  • 양막은 융모막 다음에 분화 e dichoronicity implies diamnionicity
  • monochorionic e diamnionic or monoamnionic
  • amnionicity: 6wks, counting the heartbeats within each G-sac

e must wait to visualize the number of amnions or yolk sacs to assess amnionicity

③ count the number of amnions

  • 7~8wks, CRL 8~12mm
  • Transvaginal preferred (Fig. 7-7)


④ amnionicity = the number of yolk sac

  • 난황낭은 양막 다음에 분화
  • 초음파상으로 난황낭은 양막이 보이기 2 주전에 보인다 .
    • 임신 2~3 기에서의 Chorionicity 의 결정


  • 10wks, two amnions in a diamniotic twin gestation become opposed and form an intertwine membrane


          • composition of membrane


  • dichorionic gestation: two layers of amnion, two layers of chorion
  • monochorionic, diamniotic: only two layers of amnion


          • gender of the twin

different gender: DCDA

12wks: visualization of geniatalia in 98% of singleton & in all of twin pregnancy

20~23wks: correctly determined in 98%

24~42wks: correctly determined in 95%

Before 17wks: TV sonography, 15~16wks: 97%


          • number of the placenta

all dizygotic ? dichorionic, 25% of monozygotic ? dichorionic e 80% of all twins will be dichorionic (two placentas)

Sonographic identification of a single placental “mass” does not differentiate between a monochorionic gestation and a dichorionic gestation with two contiguous fused placentas

A potential pitfall ? 1) a large succenturiate lobe with a single shared placenta ? follow the cord insertion back to a single shared placenta 2) demonstration of ant & post placental sites ? lateral uterus aspect of the uterus or across the fundus



          • interfetal dividing membrane

interfetal membrane e diamnionicity


monoamnionicity ? entanglement of the umbilical cords, conjoined twin, single umbilical cord that contains more than three vessels


before 26wks: chorionicity based on membrane thickness, 임신이 진행할수록 membrane 이 얇아진다 .


thick membrane: >1mm~2mm, well defined, hyperechoic

e predictive value, DCDA


요약 : interfetal membrane thickness may be useful in assessing chorionicity, particulary in the 1 st and early 2 nd trimesters, when the interfetal membranes tends to appear thick in DC


          • “twin peak”

== the appearance of the intertwin membrane at its junction with the placenta


In DC with fused placentas: triangular projection of trophoblastic tissue isoechoic with placenta may be identified extending from the placental surface into the intertwin membrane



Absence of the twin peak: neither excludes dichorionicity nor implies monochorionicity



    • growth in multifetal gestation


the rate of growth of twins:

  • until 28~30wks, similar to that of singletons
  • Published singleton nomogram: 32~34wks 까지는 유사 , 25~32kws 에서는 underestimate, 34wks 에서는 overestimate
  • after 30wks: BPD, AC ? 단태아보다 작다 .
  • fetal position 때문에 BPD 나 HC 를 정확히 측정하기 힘든 경우가 흔히 있다 . ? EFW 측정을 위해 AC 와 FL 를 사용


    • Discordance of fetal Wt.

출생체중의 20% 이상의 차이

discordancy 의 이유 :

  • 이란성 쌍둥이 ? 성장 잠재력의 차이
  • 기형 (anueploidy)
  • Inequality of placentation
  • in monochorionic, monozygotic: unbalanced vascular shunting

mortality: 19%


    • Cx. unique to monochorionic gestations


twin-to-twin transfusion


    • share the same placenta, placental vascular anastomosis
    • 35% 에서 ( 전 쌍둥이의 8% 에서 ), 한 쌍둥이는 polyhydramnios 가 심하고 다른 한명은 양수가 거의 없이 “ 붙어 ” 있게 된다 .
    • TOPS (twin oligohydramnios/polihydramnios sequence)
  • same sex twin
  • single placenta with a thin separating membrane
  • discordance > 20%
  • major amniotic fluid volume difference c a stuck twin

\ 4) EFW 에서 20% 이상의 차이를 보일 ‹š

  • suspect chronic TTTS in case of TOPS
    • true chronic TTTS 의 경우 , cordocetetsis 시행
    • 성장 지연된 twin 의 umbilical a. 의 S/D ratio 증가
    • The donor umblical a. and recipient v. enter and exit the same foramen on the placental surface
    • TOPS 가 있을 때 , Doppler (PI (pulsatility index) > 0.5) 는 TTTS 의 가능성이 높다 .
    • TOPS 는 TTTS 가 없을 때도 보일 수 있다 .
  • PROM
  • Fetal anomaly (Fetal renal disease, proximal GI obstruction)
    • stuck twins in TTTS ? 대부분 anterior or lateral


    • Demise of co-twin

1) “ vanishing twin”

  • 20% of twin gestation in 1st trimester

2) reduced probability of the successful delivery

  • earlier GA, monochorionicity, any sonographic abnormality (subchorionic fluid, uterine fibroids, discrepancy in size)
  • maternal age, method of conception, bleeding 은 무관

3) early loss of one twin

  • 남은 태아에 거의 영향을 미치지 않는다 .
  • 2 nd and 3 rd trimester: the death of one twin
  • significantly increased mortality and severe morbidity in the survivor
  • outcome 는 monochorionic 의 경우 더 나쁘다 .
  • 뇌 , 간 , 신장에 infarction and necrosis e major neurologic and renal impairment
    • Conjoined twin
  • frequency: 1/50,000 to 100,100
  • 대부분 조산
  • 40% 는 사산
  • monozygote, 수정 13 일 이후 embryonic disk 가 불완전하게 분리되어 발생
  • 대분분 monozygotic twin 은 male, 70% of conjoined twin 은 female
  • 진단 : fusion of fetal part 가 보일 때
  • single placenta, no visible separating amniotic membrane
  • 반대로 , two placenta, separating amniotic membrane 이 있는 경우 conjoined twin 의 진단 배제
  • 만일 진단이 불확실할 경우 CT, MRI 고려
  • 초음파 소견 :
  • 1) inseparable fetal bodies and skin contours
  • 2) no change in the relative position of the fetuses
  • 3) both fetal heads persistently at the same level
  • 4) bi-breech and, less commonly, bicephalic presentation
  • 5) single umbilical cords more than three vessels
  • 6) bifid appearance to the 1 st trimester
  • 7) fused ventrally and face eace other, resulting in backward flexion of the cervical spine
  • 8) hearts, liver, 또는 다른 organ 의 공유
  • 9) omphalocele and congenital heart disease
  • 10) polyhydramnios in 50% of conjoined twin ( 정상 twin 은 10%, 정상 단태아는 2%)


7) Pitfall

  • inseperable skin contour 는 영구적이어야 한다 .
  • diamnionic twin 사이의 양막이 안 보일 수도 있다 .
  • presentation 이 다르더라도 진단을 배제할 수 없다 .
  • conjoining 이 여러군데 생기면 단태아처럼 보인다 .


The most common types of conjoined twinning:




8) Conjoined heart: 심장이 해부학적으로 분리되거나

심박수가 다를 때 심장이 분리될 수 있다 .

  • Fetal echo 가 필요하다 .


    • Acardiac twin (twin reversed arterial perfusion) sequence
  • GA 11wks
  • severely malformed fetus, in a monochorionic twin gestation
  • monoamniotic gestation
  • diffuse edema, cystic hygroma
  • TTTS 와는 다르게 머리 , 몸통 , 상지 기형이거나 없다 .
  • acardiac twin 에서 umblical cord 의 reversed flow in Doppler
  • 임신 초기에는 fetal demise 처럼 보인다 .


    • Monoamniotic twinning
    • intertwin membrane 이 안보일 때
    • entanglement of the twin umbilical cord
    • Color Doppler
    • Two different heart rate
    • presence of conjoined twin
    • umbilical cord with more than three vessels


    • Reduction of multiple pregnancies
  • 삼태 이상 ? mortality and morbidity 감소시키기 위해
  • 정상아의 outcome 을 향상시키기 위해 , 심각하지만 치명적이지 않은 기형을 가진 다른 태아를 제거
    • Heterotopic multifetal pregnancy

불임치료를 받은 환자에서 많이 생긴다 .


ART 후 1%

모든 다태 임신은 extrauterine pregnancy 가능성에 대해 평가해야 된다 .