Embryology & Incidence of Multiples

Dizygotic

-non-identical twins

-fertilization of 2 ova by 2 sperm

-70% of all twins

-least risk of complication in twin pregnancy

Types of dizygotic twins

dichorionic / diamniotic

Monozygotic

-identical twins

-fertilization of 1 ova by 1 sperm with subsequent division into 2 embryos

-30% of all twins

types of monozygotic twins

-dichorionic/diamniotic

-monochorionic / monamniotic

-monochorion/diamniotic

chorion differentiates at

day 4

amnion differentiates at

day 8

most common type of twins

dichorionic diamniotic

dichorionic diamniotic

-each zygote develops its own amnion, chorion, and placenta

-4-layer membrane between sacs, chorion sandwiched between two amnions

-fetuses are genetically different

there are three types of monozygotic twins determined by

day of cleavage

dichorionic diamniotic twins - division occurs between

day 1-3

monochorionic diamniotic - division occurs between

day 4-8

monochorionic monoamniotic - division occurs between

day 8-13

dichorionic diamniotic

-division occurs between day 1-3 before chorion and amnion differentiate

-results in two separate zygotes (2 chorionic cavities and 2 amniotic cavities)

-4 layer membrane between sacs

-each has their own placenta

-sonographically the same as dizygotic twinning

monochorionic diamniotic

-division between day 4-8 after chorion differentiates and before amnion differentiates

-one implantation site

-results in 1 chorionic cavity and 2 amniotic cavities

-2 layer membrane, no chorion between 2 amnions

-shared placenta

what type of twinning has the highest risk for fetal mortality

monochorionic monoamniotic

monochorionic monoamniotic

-division between day 8-13 after chorion and amnion differentiate

-division of bilaminar disc just before appearance of primitive streak

-results in 1 chorionic and 1 amniotic cavity, no membrane

-shared placenta

-highest risk for fetal mortality

-cord entanglement & twin to twin transfusion

conjoined twins

-division that occurs after 13 days resulting in incomplete separation

-rare

-females > males

-mortality rates are very high

-dystocia - difficult or abnormal labor and delivery

thoracopagus

-most common classification of conjoined twins

-may share organs

omphalopagus-xiphogaus

-anterior abdominal wall conjunction

-6 vessel cord

craniopagus

lying on right angles to each other (head-to-head)

pyopagus

bottom-to-bottom

all dichorionic are

diamniotic

all monoamniotic are

monochorionic

monochorionc can be

either DA or MA depending on day of division

incidence of monozygotic twins

-constant across populations

-independent of rare, age or parity, heredity, ovulation induction and maternal genetic

incidence of dizygotic twinning

-advance maternal age

-multiparity

-post-pill pregnancy

-ovulation inducing ages/infertility

-maternal history of twinning

-african american

clinical signs and symptoms of twins

-hyperemesis

-elevated MSAFP

-large for dates

-more than one heartbeat

risk and complications of twin pregnancies

-higher raters of mortality

-prematurity - twin growth rates are similar to singletons up to 30 weeks GA

-IUGR

-placental insufficiency

fetal risks and complications in twin pregnancies

-demise of 1 or both fetuses

-congenital anomalies

-potential for cord entanglement (monoamniotic)

-arteriovenous shunting between twins (monochorionic)

-1 or 2 vessel cord

-discordant growth

-AFV diorders (poly oli)

cord entanglement is only seen in

monoamniotic

arteriovenous shunting is only seen in

monochorionic

maternal risks and complications

-pregnancy induced hypertension, edema

-PROM

-preterm labor

-3rd trimester bleeding

-anemia

-placental accidents (hemorrhage, previa, abruption)

-preeclampsia

uncomplicated DC twins deliver at

38 weeks

MC twins deliver at

36 weeks

MA twins deliver at

34 weeks

birth weight discordance

> 20%

BPD discordance

5mm or 5%

AC discordance

20 mm

difference of ____mm in __ weeks of interval growth

3mm in 2 weeks

placentation helps determine

the level of risk for anomalies, growth problems and prematurity

chorionicity

number of chorions (placentas)

amnionicity

number of amnions (yolk sac)

placentation is most accurate between

6-9 weeks

intertwin membrane may not be seen before

-10 weeks while the amnion is growing

-TV is more useful in identifying extremely thin and delicate amnion

yolk sac differentiates slightly later than the amnion, two YS indicates

two amnions

2 yolk sacs

MC DA

1 yolk sac

MC MA

placentation after 10 weeks

-gestational sac is no longer distinctly separable

-membrane is formed by 10 weeks; opposition of the two amnions

DC membrane consists of

-two layers of chorion and amnion (4 layers)

-thicker and more reflective

MC/DA membrane consists of

-two layers of amnion

-thin and wispy

-less reflective

twin peak

-presence of placental tissue projecting between membranes of the chorion

-occurs only in DC

T-sign

single chorion serves as a barrier removing the potential space between intertwin membranes

no membrane could mean

MC/MA

-stuck twin with complete oligohydramnios

-normal pregnancy usually MC/DA in which membrane is present and not seen

conjoined twin criteria

-cannot separate twins

-no membrane

-complex structural anomalies

-> 3 vessel cord

-US helps determine the extend to which the organs are shared

-sharing of heart or brain excludes successful separation

vanishing twin phenomenon

-71% loss of one twin during pregnancy

-cardiac activity used as a means to diagnose in 1st trimester

vanishing twin phenomenon is due to

-resorption of GS

-blighted ovum

-fetus papyraceous - paper doll

-technical erros