MULTIPLE PREGNANCY.doc - MULTIPLE PREGNANCY TYPES Either Monozygotic(MZ or Dizygotic(DZ Monozygotic(MZ a single fertilized ovum splits into two distinct


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MULTIPLE PREGNANCY TYPES Either Monozygotic (MZ ) or Dizygotic (DZ) : Monozygotic (MZ ) : a single fertilized ovum splits into two distinct individuals after a variable number of divisions. Such twins are almost always genetically identical and of the same sex. On rare occasions, mutations can cause genetic discordance resulting in phenotypic and chromosomal disimilarities between MZ twins. The frequency of MZ twins is fairly constant throughout the world at a rate of approximately 4 per 1,000 births. This rate is unrelated to maternal age, race, or parity. The cause of MZ twinning is unclear Dizygotic (DZ) : Two separate ova are fertilized . These individuals are genetically distinct -that is DZ twins are genetically dissimilar true siblings. They may have the same or opposite sex. DZ twinning is associated with multiple ovulation and its frequency varies between races and within countries and is affected by several identifiable factors. In general, the frequency of DZ twins is low in Asians, intermediate in whites, and high in blacks. The Yorubas of western Nigeria have a frequency of 45 twins per 1,000 births, and about 90 percent are DZ. The frequency of DZ births is affected by maternal age. The frequency of DZ twinning also increases with parity independent of maternal age. PLACENTATION Twin placentas are described in terms of their membranes . The sac of a singleton pregnancy consists of an outer chorion and an inner amnion. Each DZ twin develops within a similar sac because both blastocysts generate their own placentas. Two separate placentas will result, each of which will have a chorion and an amnion. Should they implant side by side, intimate fusion of the placental disks will occur, but these placentas are always diamniotic and dichorionic. Vascular anastomoses rarely occur . While MZ twins may also have placentas with two amnions and two chorions, this generally is not the case. Monochorionic placentas have a single chorion that usually surrounds two amnions, but occasionally there is only a single amnion. Regardless of whether these monochorial placentas are diamniotic or monoamniotic, they are always a single disk and only occur in MZ twins . Triplets and quadruplets have also been delivered with monochorial placentas and shown to be MZ. Almost all monochorial placentas have blood vessel communications between the fetal circulations. Monochorial placentas can only occur in MZ pregnancies 1
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The type of placenta that develops in an MZ pregnancy is determined by the timing of cleavage of the fertilized ovum. If twinning is accomplished: During the first 2 to 3 days , it precedes the setting aside of cells that eventually become the chorion. In that case, two chorions and two amnions will be formed. After 4 days , however, twinning cannot split the chorionic cavity, and from that time on a monochorial placenta must result.
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