4. Zygosity and vanishing twins Flashcards
What causes dizygotic twins?
2 eggs fertilised by 2 sperm
Incidence increases with maternal age
Mutations in GDF9 associated with twinning - TGF9 signalling pathway regulates multiple follicle growth and multiple ovulation rate
What causes monozygotic twins
1 fertilised egg splits to form 2 embryos
Rarely familial, no age effect
What are the three types of monozygotic twins?
When do they occur?
1/3 are dichorionic/diamniotic (DC/DA) - division before/during morula stage (day 4) - separate placentas
2/3 are monochorionic/diamniotic (MC/DA) - division of ICM at blastocyst stage (day 4-7) - single placenta with 2 amnions
Minority are monochorionic/monoamniotic (MC/MA) - division after formation of amnion, sometimes conjoined
What problems are associated with twinning?
Increased risk of perinatal mortality, preterm delivery, IUGR, congenital abnormalities (hydrocephaly, heart defects)
Increased complications with monochorionic twins
What is TRAP?
Twin reversed arterial perfusion
Monochorionic only, cardiac system of one twin supplied blood for both
Acardiac twin doesn’t develop properly, ‘pump’ twin has poor prognosis
What is TTTS?
Twin to twin transfusion syndrome
Placental vascular anastomoses connect artery of one twin to vein of second - unidirectional blood flow
Causes asymmetric growth, mortality in 80% cases
Laser ablation of placental anastomoses
What is a vanishing twin?
When one twin dies early and isn’t aborted
Vanishing twin - represented by areas of degenerate villi or amorphous debris
Fetus papyraceous - dead fetus persists (death after first trimester) due to chromosomal abnormality
What is zygosity testing?
Why is it used in twin studies?
Degree of identify in the genomes of twins
When one twin develops phenotype with possible genetic cause - determine if twins are monozygotic to establish recurrence risk in twin
What are the prior risks of monoygosity?
3/10 monozygotic, 7/10 dizygotic
2/5 and 3/5 if gender of both twins is known
