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Flashcards in Twins Deck (61)
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What is the rate of loss at term of monochorionic twins?

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

MC twins: 8%

DC twins: 2%


What percentage of monochorionic twins are affected by TTTS?



What is the definition of zygosity?

1) The genetic make-up/character of a particular zygote

2) The genetic relationship between offspring of a twin or multiple pregnancy.


What is the definition of chorionicity?

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


Describe the zygosity of dizygotic twins

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


Describe the mechanism of dizygotic twins

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.


Describe the mechanism of monozygotic twins

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


What increases the rate of monozygotic twins

IVF and ovulation induction


How is MZ twin chorionicity determined?

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.


When is the optimal time to determine chorionicity?

<14 weeks gestation


What is the best way to date a twin pregnancy?

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


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

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%


What percentage of all twins are monozygotic?



Regarding combined maternal serum screening:

How is risk calculated for DC twins?

How is risk calculated for MC twins?

  • 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).


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

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.


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

  • 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%


Monochorionic twins should have fortnightly scans from what gestation?

What should be assessed on ultrasound?

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).


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

  • 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.


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

  • 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.


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



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

>/= 25% discordance


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

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


What is the criteria for Quintero Stage I of TTTS

Significant discordance in amniotic fluid volumes:

Oligohydramnios with DVP <2cm

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


What is the criteria for Quintero Stage II of TTTS

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


What is the criteria for Quintero Stage III of TTTS

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


What is the criteria for Quintero Stage IV of TTTS

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


What is the criteria for Quintero Stage V of TTTS

One or both babies have died

Not amenable to therapy


What is the risk of TAPS in uncomplicated monochorionic twins?

5% (RANZCOG) 2% (RCOG)


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

10% (RANZCOG) 13% (RCOG)


What is the diagnostic criteria for TAPS?

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)