L3 Multiple Pregnancy Flashcards
Define zygosity:
- Original number of fertilised eggs
- e.g. monozygous twins have split from one original egg
Define chorionicity:
- Whether the placenta is shared or not
- e.g. dichorionic twins do not share a placenta
Mono- vs diamniotic twins:
- Diamniotic: both babies share the same amniotic sac
- e.g. MCMA means monochorionic monoamniotic where everything is shared
What shape would you look for on an USS to distinguish chorionicity?
- Lambda sign: dichorionic
- T-shape: monochorionic
- Other signals: Membrane thickness
At what points does monozygotic typically occur following fertilisation (and in what proportion)?
- Days 0 -3: Splitting of morula, ~25%
- Days 4 - 7: Split at hatching, ~75%
- Days 7 - 14: Blastocyst splitting after implantation, 1 - 2%
Describe Helin’s law:
- Rule of thumb for incidence of multiple pregnancies
- 1 in 89^n-1 twins where n = parity
- e.g. Twins 1 in 89 pregnancies where n = 2
How does incidence of dizygotic twins vary around the world?
- 6 in 1000 in Asia
- 10 -20 in 1000 in Europe/USA
- 40 in 1000 in Africa (Highest in Nigeria)
List some key risk factors for DZ twins:
- ART (ovarian stimulation, mET)
- Maternal age
- Parity
- Genetics (multiple modes of inheritance described)
- Dietary sources of oestrogen
- Geography
- Seasonal light
What is the logic behind most risk factors for DZ twins?
- Multiple ovulation
- Two eggs reaching maturity at the same time and being fertilised by 2 sperm
How did the sET policy in ART affect multiple births?
- Aiming to reach a target of 10% multiple births
- Policy implemented in 2007 (UK)
- 13% elective single transfer in 1991 vs 75% in 2019
- Foudn to have same pregnancy rate as mET but with lower multiple pregnancy rate
What is known about risk factors for MZ twins?
- Mechanisms largely unclear
- No natural animal models exist to study this type of twinning
- Factors related to environment are likely to include hypoxia, delayed implantation and temperature
- Definitely known to be more common with IVF -> potentially via disruption of ZP, temperature and blastocyst transfer stage
What complications can arise in cases of monochorionic twinning?
- Twin-to-twin transfusion syndrome (TTTS)
- Twin anaemia polycthaemia sequence (TAPS)
- Twin reverse arterial perfusion sequence (TRAPS)
- Selective fetal growth resriction
- Cord entanglement (MCMA)
What complications can affect all types of twins?
- Maternal: Anaemia, GTT (Diabetes), PIH/PET (hypertension), hyperemesis, PPH
- Fetal: Stillbirth, preterm births, miscarriage (3x), growth restriction, fetal abnormalities
Clinical management of different types of twins (during gestation):
- GA and chorionicity determined at 11-14w -> nomenclature assigned
- All types: 12w NT scan
- Scans 4 weekly for DCDA and 2 weekly for MCDA and MCMA
- Surveillance should take place for TTTS as well as maternal risks
- Regular growth scans
When might delivery be induced for different types of twins?
- DCDA: 37 - 38 weeks
- MCDA: 36-37 weeks
- MCMA: 32 weeks
How is the risk of congenital abnormalities affected in twins?
- Dichorionic: same risk as normal
- Monochorionic: 2-3x increased rate of CAs (20% chance of both twins being affected)
- Management depends on nature of abnormality
What type of congenital abnormalities can be seen in monochorionic twins?
- Malformations e.g. neural tube defects, congenital heart disease (possibly due to part of twinning process)
- Disruptions (e.g. limb reduction defects)
- Deformations (e.g. club foot, hip dysplasia) -> overcrowding
How will twins typically be delivered:
- MCMA: C-section at 32 weeks
- MCDA/DCDA: Varies based on GA, evidence of fetal compromise, maternal history and preference
What is TTTS?
- Twin to twin transfusion syndrome
- In monochorionic pregnancies, vascular anastomoses occur between the two ‘halves’
- Some are bidirectional (A-A, V-V) and thus are superficial
- Some are unidirectional (particularly worried about deep A-V anastomoses)
- This transmits one-way flow from a donor to recipient twin
- Occurs in 15% of monochorionic twins with dire consequences for both babies
How is TTTS diagnosed?
- USS (2 weekly from 16 weeks)
- Looking at liquor volumes, stomach and bladders of babies, doppler, growth
- Maternal: shortness of breath, increased abdominal girth
How is TTTS graded?
- Grades I to V increasing in severity
- Lowest: Significant discordance in amniotic fluid volumes (specific threshold for DVP aka deepest vertical pocket) but normal doppler and donor bladder visible
- Highest: One or both babies dead
- Will typically be treated at grade I or II
How is TTTS treated?
- Fetoscopic laser ablation
- Improved survival compared to amnioreduction
- Ongoing USS monitoring after procedure
- Delivery at 34 - 37 weeks
What is TAPS?
- Twin anaemia-polycythaemia sequence
- Small a-v vascular anastomoses (can occur following laser for TTTS)
- Results in anaemia-polycythaemia discordance without haemodynamic instability (i.e. imbalance in RBCs)
How is TAPS screened and managed?
- MCA peak systolic velocity
- Management uncertain; expectant delivery, transfusion, selective fetocide, repeat laser
What occurs in TRAPS?
- Twin reversed arterial perfusion
- One twin supports an abnormal heartless tissue mass (acardiac twin)
- 1% monochorionic twins
- Acardiac twin certain to die, pump twin at 50% mortality (heart failure, preterm birth, hypoxia)
What are the risks to the remaining twin if one dies in utero?
- 15% mortality
- 26% neurological abnormality
- 66% preterm birth
- Essentially due to the survivor losing blood into the dying twin
What are the risks associated with MCMA twins?
- Very high risk; 60% overall survival rate
- Occurs in 1% of all twins
- Cord entanglement!
- Delivery by CS at 32 weeks
What are the neonatal morbidities associated with spontaneous preterm birth?
- Acute: Respiratory, gastrointestinal, neurological
- Chronic: neurodevelopment, mental, respiratory, growth
