Obstetrics & Gynecology>>>>>Maternal-Fetal Physiology and Placentation
Question 1#

A 29-year-old Caucasian primigravida is 20 weeks pregnant with twins. Today, on her routine ultrasound for fetal anatomy, she found out that she is carrying two boys.

In this patient’s case, which of the following statements correctly describes the zygosity of this pregnancy?

a. The twins must be monozygotic since they are both the same gender
b. If division of these twins occurred after formation of the embryonic disk, the twins will be conjoined
c. She has a higher incidence of having monozygotic twins because she is Caucasian
d. If the ultrasound showed two separate placentas, the twins must be dizygotic
e. If the ultrasound showed two separate placentas, the twins cannot be monozygotic

Correct Answer is B


The incidence of monozygotic twinning is constant at a rate of one set per 250 births around the world. It is unaffected by race, heredity, age, parity, or infertility agents. The incidence of dizygotic twinning is influenced by all of these factors, and varies based on group. These twins of the same gender could be monozygotic or dizygotic. Two identifiable chorions can occur in monozygotic or dizygotic twinning. Dizygotic twins will always have two amnions and two chorions, since they result from fertilization of two eggs. Therefore, dizygotic twins may be of the same or different genders. The placentas of dizygotic twins may be totally separate, or intimately fused, depending on the location of the implantation of the two zygotes. Monozygotic twins are always of the same gender because they originate from the division of one zygote; however, they may be monochorionic or dichorionic depending on when the separation of the twins occurred. Twenty to thirty percent of monozygotic twins have dichorionic, diamniotic placentation (similar to dizygotic twins), which results from separation of the blastocyst within the first 72 hours after fertilization. Division that occurs between days 4 and 8 will result in monochorionic, diamniotic twins. One percent of monozygotic twins will be monochorionic, monoamniotic, which occurs with division after day 8 but before the embryonic disc is formed. Conjoined twins are always monozygotic, and occur with late division after formation of the embryonic disk.