Pathology of Twinning Flashcards
Dizygotic vs monozygotic twins
Dizygotic (“Fraternal”): 2 ova fertilized by 2 sperm Not genetically identical ~70% of spontaneous twins ~95% of Assistive Reproductive Technology twins
Monozygotic (“Identical”) 1 ovum fertilized by 1 sperm, fertilized oocyte divides Genetically identical ~30% of spontaneous twins 3-5/1000 births
When embryo divides and how that affects the growth of twins?
When splits:
1-3 days: Dichorionic, diamniotic twins (monozygotic or dizygotic) 20-30% of monozygotic twins
4-8 days: Monochorionic, diamniotic twins (monozygotic ONLY) 70% of monozygotic twins
9-12 days: Monochorionic, monoamniotic (monozygotic ONLY) 1% of monozygotic twins. More dangerous (umbilical cords can get wrapped around each other)
13-15 days: Conjoined twins, monochorionic, monoamniotic
How do we determine if twins are di or mono chorionic and amniotic?
Ultrasound
Dichorionic/Diamniotic: thick dividing membrane; “twin peak” or “lambda” sign
Monochorionic/Diamniotic: thin dividing membrane; “T” sign
What are the layers of the placenta starting from baby to uterine wall
Amnion, chorion, extravillous trophoblast, decidua capsularis
Amnion
single layer of flat, cuboidal or columnar epithelial cells derived from fetal ectoderm. Passively attached to chorion by amniotic fluid pressure.
Amniotic Fluid: Absorbs jolts, allows fetal movement, sustains amnion and is swallowed by fetus after 20 weeks GA; aids lung development
Chorion
tough fibrous layer that carries fetal blood vessels, often with atrophied villous remains (chorion frondosum).
Why Are Twin Rates Increasing?
20x higher incidence with fertility treatment
25% of Assistive Reproductive Technology (ART) pregnancies worldwide are twins
Only ~50% of twin pregnancies are conceived spontaneously
Also ↑ with maternal age independent of ART
Risks of Twin Pregnancies
Miscarriage
Hyperemesis (higher levels of hCG)
Increased risk of aneuploidy and anomalies (increased 3-5x in monozygotes)
Prenatal screening tests less sensitive and diagnostic
procedures more difficult
Maternal anemia
Gestational diabetes (? increased hPL)
Gestational hypertension/preeclampsia (increased 2x)
Intrauterine growth restriction
Preterm birth – $$$
Cesarean delivery
Postpartum hemorrhage
Higher perinatal mortality – 5-7x rate in singletons
Why does it matter what kinds of twins you have?
Affects the complication rate and mortality of the twins
monoamniotic/monochorionic are most dangerous
Twin-Twin Transfusion Syndrome
TTTS
Monochorionic-diamniotic twins that are MONOZYGOTIC by definition
15-20% of monochorionic-diamnionic twins have unbalanced flow through connected vessels
Recipient twin (larger) increases urine production to reduce blood volume: Large bladder on ultrasound abd polyhydramnios
Donor twin (smaller) reduces urine production to retain blood volume- Oligohydramnios
Classification based on ultrasound findings
Who is most at risk for TTTS and why?
monochorionic-diamnionic twins
More likely to have unbalanced arterio-venous connections
Dichorionic twins shouldn’t have any connecting vessels
Monoamniotic twins have so many vascular connections, they tend to balance out
Prognosis of TTTS
Untreated, TTTS prior to 24 weeks gestational age leads to mortality of one or both twins in 80-90% of cases
After death of one twin, other twin at increased risk for brain damage in 1/3 of cases
Severe TTTS prior to 16 weeks has dismal prognosis
What causes death of donor twin
Decreased blood volume
Lower urine output
oligohydramnios
Small placental volume; not enough nutrients
What causes death of recipient twin
Too much blood volume, strains baby heart
Too much urine production:
Polyhydramnios
Fetal hydrops
Treatment options for TTTS
Reduction amniocentesis
Microseptostomy
Laser Ablation
