78 Obstetrics and Gynecology Board Review •••
❍ What is the difference between chimera and mosaicism?
Chimera: Individual whose cells originated from more than on fertilized ovum
Mosaicism: Two or more cell lines from different chromosomal composition arise from the same zygote as a
consequence of nondisjunction
❍ How does the timing of division of the fertilized ovum affect monozygotic twins?
≤3 days after fertilization: Diamniotic, dichorionic
4 to 8 days: Diamniotic, monochorionic
8 to 13 days: Monoamniotic, monochorionic
13 days: Conjoined twins
❍ How does examination of the placenta aid in the determination of zygosity of like-sex twins?
Evaluation of the amnion(s) and chorion(s).
❍ Other than opposite sex, what is the most predictive way to diagnose mono versus dichorionic gestations
via ultrasound?
Membrane character and thickness.
Dichorionic: The dividing membrane is >2 mm with 3 to 4 layers identified and a “twin peak” sign seen on
ultrasound, where the chorion attaches to the amnion
Monochorionic: The dividing membrane is <2 mm and “hair-like” with only two layers and a “T” sign is noted
where the chorion and amnion attach
❍ What type of monozygotic twinning is most common?
Monochorionic–diamniotic.
❍ Which type of twinning has the highest morbidity?
Monoamniotic–monochorionic.
❍ Which type of twin placenta is at highest risk of vascular communications?
Monochorionic.
❍ What is the perinatal mortality rate of dichorionic-monoamniotic twins?
25%.
❍ What is the mortality rate of mono-mono twins?
50% to 60%.
❍ What is the recommended method of delivery for mono-mono twins?
Cesarean delivery.
❍ What is the biggest mortality risk to mono-mono twins?
Cord entanglement.