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General obstetrics > Twins > Flashcards

Twins Flashcards

1
Q

What is the rate of loss at term of monochorionic twins?

What is the rate of loss at term of dichorionic twins?

A

MC twins: 8%

DC twins: 2%

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2
Q

What percentage of monochorionic twins are affected by TTTS?

A

10-15%

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3
Q

What is the definition of zygosity?

A

1) The genetic make-up/character of a particular zygote
2) The genetic relationship between offspring of a twin or multiple pregnancy.

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4
Q

What is the definition of chorionicity?

A

The number of chorion (outer membrane) and placenta in a multiple pregnancy.

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5
Q

Describe the zygosity of dizygotic twins

A

Result from the fertilisation of two different eggs by two separate sperm. Therefore genetically non-identical.

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6
Q

Describe the mechanism of dizygotic twins

A

Results from multiple ovulation. Is associated with higher maternal FSH and is influenced by with season, geography, maternal age (higher age >40), ethnicity (subsaharan african) and body habitus.

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7
Q

Describe the mechanism of monozygotic twins

A

Result from the fertilisation of one egg by one sperm and subsequent splitting of the fertilised egg. Therefore genetically identical.

By timing of embryo division:

Day 1-3 = DCDA (25%)

Day 4-7 MCDA (70-75% )

Day 8-12 MCMA (1-2%)

>= Day 13 conjoined very rare

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8
Q

What increases the rate of monozygotic twins

A

IVF and ovulation induction

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9
Q

How is MZ twin chorionicity determined?

A

Firt trimester USS by 14 weeks, to assess for T (monochorionic/diamniotic) or Lambda/twin-peak sign (dichorionic).

After 14 weeks the chorion and amnion fuse making diagnosis harder. After this time sex discordance between twins is the only way to determine it, as a male and female fetus = dizygotic and thus DCDA twins.

MCMA pregnancies have no visible membrane between fetuses, cords may look tangled, and ‘galloping horse’ sign when spectral doppler applied to loops of cord to obtain flow velocity waveform.

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10
Q

When is the optimal time to determine chorionicity?

A

<14 weeks gestation

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11
Q

What is the best way to date a twin pregnancy?

A

Dating scan prior to 14 weeks. Optimal dating by CRL between 10+0 to 12+6 weeks. From 13+0 use HC (>80 mm)

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12
Q

Describe the ultrasonographic features used to determine dichorionic twins on early scan

A

Lambda or twin peak sign: triangular projection of tissue that extends beyond the chorionic surface of the placenta and is wider at the chorionic surface. May only be used in 1st trimester but has specificity 100%

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13
Q

What percentage of all twins are monozygotic?

A

30%

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14
Q

Regarding combined maternal serum screening:

How is risk calculated for DC twins?

How is risk calculated for MC twins?

A
  • DC twins:
    • Each twin’s NT is measured and combined with serum analytes.
    • Can calculate risk for each twin or pregnancy specific risk (sum of both twins).
  • MC twins:
    • Mean NT is used and combined with serum analytes
    • Calculates risk of aneuploidy for both twins (as they are genetically identical).
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15
Q

What is the false positive rate for combined 1st trimester screening (CFTS) / MSS1 in - singletons - dichorionic twins - monochorionic twins

A

Singleton: 2.5%

Dichorionic: 5%

Monochorionic: 10%

NB. CFTS/MSS1 has much lower detction rate in twin pregnancies, thus some centres will use NT alone without maternal biochemistry, or recommend NIPT instead.

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16
Q

Discuss the utility of MSS-2 screening for aneuploidy in twin pregnancies:

A
  • Can be used if missed MSS-1
  • Only able to calculate risk for T21
  • Sensitivity lower than MSS-1:
    • DC twin: 30%
    • MC twins: 80%,
  • False positive rate for MC twins 3%
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17
Q

Monochorionic twins should have fortnightly scans from what gestation?

What should be assessed on ultrasound?

A

From 16/40

Fetal growth, amniotic fluid volume in both sacs, fetal bladder volume.

From 20/40 UAPI and MCA PSV should be routinely assessed. (This may start earlier if abnormalities arise from an earlier gestation).

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18
Q

Discuss the utility of combined maternal serum screening for twin pregnancies:

A
  • High sensitivity 90% but less sensitive than for singleton pregnancies.
    • Assumes equal contribution of biochemical markers but this is unlikely if discordant chromosomes i.e.g DC twins.
  • Higher false positive rate especially for MC twins.
    • Because increased NT can reflect early manifestation of other complications e.g. TTTS.
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19
Q

Discuss the utility of NIPT for aneuploidy screening in twin pregnancies:

A
  • Good option for twins:
    • T21 sensitivity 99%
    • T18 sensitivity 85%
    • False positive rate 0%
  • If MZ twins probably similar sensitivity to singleton.
  • Higher failed test rate 5% cf. 1.7% in singletons
    • Due to lower fetal fraction.
  • Not funded but option should still be discussed with patients.
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20
Q

Monochorionic twins should have fortnightly growth scans. When should the UAPI, MCA PI, PSV and CPR be measured from?

A

20/40

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21
Q

An EFW discordance of how much, is associated with an increased perinatal risk?

A

>/= 25% discordance

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22
Q

What symptoms should mothers of Monochorionic pregnancies specifically be advised to monitor for?

A

Increased abdominal girth and shortness of breath; this may be manifestation of polyhydramnios as a result of TTS

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23
Q

What is the criteria for Quintero Stage I of TTTS

A

Significant discordance in amniotic fluid volumes:

Oligohydramnios with DVP <2cm

Polyhydramnios with DVP > 8cm, or DVP > 10cm if GA > 20 weeks

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24
Q

What is the criteria for Quintero Stage II of TTTS

A

Bladder of the donor twin is not visible over 60 mins and severe oligohydramnios due to anuria

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25
Q

What is the criteria for Quintero Stage III of TTTS

A

Doppler studies are critically abnormal (UAPI absent or reversed EDF, reversed DV) in either twin

26
Q

What is the criteria for Quintero Stage IV of TTTS

A

Ascites, pericardial or pleural effusion, scalp oedema or overt hydrops present, usually in the recipient twin.

27
Q

What is the criteria for Quintero Stage V of TTTS

A

One or both babies have died

Not amenable to therapy

28
Q

What is the risk of TAPS in uncomplicated monochorionic twins?

A

5% (RANZCOG) 2% (RCOG)

29
Q

What is the risk of TAPS, following laser ablation for TTTS

A

10% (RANZCOG) 13% (RCOG)

30
Q

What is the diagnostic criteria for TAPS?

A

Discordance in MCA PSV

Donor twin has MCA PSV > 1.5 MoM

Recipient twin has MCA PSV < 1.0 MoM (RCOG guideline, some others say <0.8MoM)

31
Q

Regarding MC twins:

What is the risk of selective FGR in the absence of TTTS?

A

15%

32
Q

What is the risk of selective FGR in the presence of TTTS?

A

Up to 50%

33
Q

In MCDA twins: if one twin dies, what is the risk of death in the co-twin?

A

15%

34
Q

In MCDA twins: if one twin dies, what is the risk of neurological abnormality in the co-twin?

A

20-30%

35
Q

What percentage of monochorionic twins are monoamniotic?

A

1%

36
Q

What is the rate of conjoined twins?

A

1:90,000 - 100,000

37
Q

What supplementation should women receive in twin pregnancies?

A

Iron supplementation

High dose folic acid (5mg)

Aspirin if any other risk factors for PET (nullip, age > 39, BMI > 34, family history of PET, interpregnancy interval > 10 years)

38
Q

At what gestation is laser photocoagulation done for TTTS

A

16-26/40

Not feasible prior to 16/40, and consideration should be given to TOP.

Generally after 26/40, amnioreduction or delivery is preferable due to the risks of the procedure.

39
Q

What are the risks of aminoreduction in context of TTTS?

A
  • PPROM
  • Placental abruption
  • Infection
  • May need serial procedures which increases risk of complications
40
Q

What does amnioreduction help with in the context of TTTS?

A
  • Reduces incidence of preterm labour
  • Reduces complications of polyhydramnios
  • Improves uteroplacental perfusion
  • BUT does not remedy underlying cause of TTS
41
Q

What percentage of monochorionic pregnancies are affected by TRAP?

A

1-5%

42
Q

How often should DCDA twins have growth scans?

A

Every four weeks from 24/40 (NICE guideline)

43
Q

What is the risk of spontaneous PTB in twins?

A

60%

44
Q

What is the risk of spontaneous preterm birth <35/40 in triplets?

A

75%

45
Q

For DCDA twins, at what gestation should you offer planned birth?

A

37-38/40

  • Continuing the pregnancy beyond 37+6/40 increases the risk of fetal death
  • Planned birth from 37+0 does not appear to be associated with an increased risk of serious neonatal adverse outcomes
46
Q

For MCDA twins, at what gestation should you offer planned birth?

A

36/40

47
Q

For MCMA twins, at what gestation should you offer planned birth?

A
  • Between 32+0 and 33+6/40
  • Continuing the pregnancy beyond 33+6/40 increases the risk of fetal death
48
Q

For TCTA or TCDA triplets, at what gestation should you offer planned birth?

A

35/40 Continuing the pregnancy beyond 35+6/40 increases the risk of fetal death

49
Q

What are the risks of selective reduction (termination) in twin pregnancies?

A
  • Loss of co-twin 15-18%
  • Acute feto-fetal transfusion neurological sequelae 15%
  • PPROM 10-15%
  • Chorioamnionitis
50
Q

What is the risk of low BW and very low BW in twin pregnancies?

A
  • Low BW 60%
  • Very low BW 10%
51
Q

What is the risk of fetal death for MCMA twins?

A

10%

52
Q

Regarding MC twins with sFGR:

Why is the latency period between abnormal UAPI and delivery longer than for singleton or DC twins?

A

Placental anastomses may paradoxically benefit the smaller twin by providing rescue transfusion of oxygenated blood from the larger twin.

53
Q

What is the Gratacos classification for?

Outline the different stages.

A

Gratacos classification system for sIUGR: based on umbilical artery doppler findings.

  • Type I: normal doppler or raised but forward flow
  • Type II: persistently absent or reversed end-diastolic flow
  • Type III: intermittent absent or reversed end-diastolic flow
54
Q

Describe the pathophysiology of TRAPS:

A

Abnormal acardiac twin with direct umbilical artery to umbilical artery anastomosis with normal pump twin. Acardic twin is non-viable.

Reverse flow of deoxygenated blood from pump twin to placenta and then into acardiac twin bypassing placental capillary bed. Deoxygenated blood preferentially perfuses acardiac twin lower body resulting in bizarre anomalies.

Can lead to high output cardiac failure, polyhydramnios and hydrops and preterm birth in pump twin with risk of pump twin mortality 50% if untreated.

55
Q

What are the poor prognostic features of TRAPS?

A
  • Continuing growth of acardiac twin
    • ratio of EFW of acardic twin to pump twin >0.7
    • AC of acardiac twin equal or greater than pump twin ratio > 1.0
  • Hydrops
  • Polyhydramnios
  • abnormal MCA PSV in donor twin shows anaemia; absent/reversed EDF UAPI or abnormal DV are signs imminent fetal demise in donor twin
56
Q

Regarding TRAPS:

What is the approach to management?

A
  • 2 weekly scans from 16/40, including MCA PSV
  • Conservative management: if EFW <50% and absence of poor prognostic indicators.
    • Weekly fetal ECHO surveillance.
  • Umbilical cord occlusion of acardiac twin: if poor prognostic indicators and not close to term.
  • Delivery: if cardiac decompensation at viable gestation.
57
Q

Regarding MCMA twins:

What is the risk of perinatal mortality %?

Why is it so high?

A
  • Perinatal mortality 10-40%
  • Due to:
    • High rates of congenital anomalies 20%
    • Cord entanglement
    • Usual MC twin risks
58
Q

What is the pathophysiology of TTS?

A
  • Unidirectional A-V anastomoses deep in placenta
  • One twin becomes donor, the other recipient
  • Donor twin becomes hypovolemic, which stimulates renin-angiotensin-aldosterone pathway, causing vasoconstriction, exacerbating FGR and oligohydramnios due to poor perfusion of kidneys
  • Recipient twin becomes fluid overloaded and develops cardiac failure, hydrops and secretes ANP causing diuresis and polyhydramnios.
59
Q

What is the prognosis for TTS?

A
  • Mod-severe TTS has 90% mortality if untreated
  • Surviving twin at risk of long term neurological, cardiac and renal morbidity
  • Survival >70% if fetoscopic laser ablation of fetal vessel anastomes
60
Q

Compare TOPS and TAPS.

A

Both on spectrum of TTS.

TOPS

  • ‘classical TTS’ - oligo/polyhydramnios
  • 10% MC twins
  • Onset usually before 24 weeks

TAPS

  • anaemia/polycythemia
  • 5% MC twins
  • Complicates 10% cases after lazer ablation for TOPS
  • Onset usually later in pregnancy
61
Q

What is the ‘solomon technique’? What is its main benefit?

A
  • Fetoscopic laser ablation technique - results in dichorionising (i.e. totally dividing placenta) rather than focussed ablation of intertwin communicating vessels
  • Reduces risk of developing TAPS after laser ablation

General obstetrics (34 decks)

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