2 mm due to 2 separate chorionic sacs + 2 separate amniotic sacs (92% accurate for dichorionic diamniotic twins)# sign = triangular projection of placental tissue insinuated between layers of intertwin membraneMonochorionic Diamniotic Twins (69-80%)# = separation in blastocyst stage between 4th and 7th day after fertilization (chorion already developed and separated from embryo)# 2 separate amniotic sacs in single chorionic sac# Common monochorionic placenta has vascular communications in 100%Cx:(1) Twin-twin transfusion syndrome(2) Twin embolization syndrome = DIC in surviving twin from transfer of thromboplastin; 17% morbidity / mortality of survivor after fetal death of twin(3) Acardiac parabiotic twinMonochorionic Monoamniotic Twins (1%)# = division of embryonic disk between 8th and 12th day after fertilization (amniotic cavity already developed) common amniotic + chorionic sac, no separating membraneDays after FertilizationEmbryologic EventCleavage Chorionresults in Amnion1-2Cell divisions - moruladi~di~3-4Chorionic differentiation 6Blastocyst implants in endometriummono'"di~8Amnionic differentiationmono'"mono'">13Division of embryonic diskmono'"mono'"# Entanglement of cords (the only definitive positive sonographic sign of monoamnionicity)Cx: double perinatal mortality up to 45%(1) Entangled umbilical cord (70%)(2) True knot of cord(3) Conjoined twins (umbilical cord with >3 vessels, shared fetal organs, continuous fetal skin contour)Prognosis: 40% survival rateDizygotic Twins (2/3)# = FRATERNAL TWINS# fertilization of two ova by two separate spermatozoa during two simultaneous ovulations (occurring either in both ovaries or in one ovary)# superfetation = fertilization of two ova by two separate spermatozoa during two subsequent ovulations (frequency unknown)# superfecundation = two ova fertilized by two different fathers (very rare)# Incidence: 1:80 to 1:90 births# Predisposing factors:- Advanced maternal age (increased up to age 35): reduced gonadal-hypothalamic feedback with increase of FSH levels- Ovulation-inducing agents (multiple pregnancies in 6-17% with clomiphene, in 18-53% with Pergonal)- Maternal history of twinning (3 times as frequent compared with normal population)- Increased parity- Maternal obesity- Race with inherited predisposition for multiple ovulations (Blacks > Whites > Asians) different phenotypes; same/ opposite sex always dichorionic diamnioticOverview of the incidence of twin pregnancy Zygosity & corresponding twin-specific complicationsTwin-Specific Complication (percent)Type of TwinningTwinsFetal Growth RestrictionPreterm DeliveryPlacental Vascular AnastomosisPerinatal MortalityDizygotic802540010-12Monozygotic204050 15-18Diamnionic/ dichorionic6-73040018-20Diamnionic/ monochorionic13-14506010030-40Monoamnionic/ monochorionic< 14060-7080-9058-60Conjoined0.002-0.008--70-8010070-90TWIN-TO-TWIN TRANSFUSION SYNDROME# In this syndrome, blood is transfused from a donor twin to its recipient sibling such that the donor becomes anemic and its growth may be restricted, whereas the recipient becomes polycythemic and may develop circulatory overload manifesting as hydrops.# The donor twin is pale, and its recipient sibling is plethoric.# Similarly, one portion of the placenta often appears pale compared with the rest of the placenta.# Complications: The neonatal period may be complicated by circulatory overload with:- Heart failure- Occlusive thrombosis.- Polycythemia may lead to severe hyperbilirubinemia and kemicterus.">