Chapter 30. Multiple Pregnancies - Review of Medical Embryology Book

Twinning may originate from 2 zygotes (dizygotic, nonidentical, or fraternal) or from 1 zygote (monozygotic, identical). Fetal membranes and placenta(s) vary with the derivation of the twins or the time when twinning occurs; e.g., should the embryo duplication occur after the amniotic cavity forms (about day 8), the embryos will be in the same chorionic and amniotic sacs

TWINS occur about once in 80-90 pregnancies (about 1%) and two-thirds or 70% are dizygotic

Dizygotic twinning shows racial differences; the incidence of monozygotic twinning, however, is the same in all populations
The rate of monozygotic twinning shows little variation with the mother's age; dizygotic twinning increases with maternal age
The anastomoses between blood vessels of fused placentas of human dizygotic twins may occur and result in erythrocyte mosaicism (red cells of 2 different types)

In cattle, the above results in a freemartin
In man, if one fetus is male and the other female, masculinization of the female fetus does not occur, but a chimera is created (has blood cells of 2 genotypes)

DIZYGOTIC (DIOVULAR AND DIVITELLINE) TWINS result from fertilization of 2 oocytes by different sperms

The twins may be of the same or different sexes
The offspring are no more alike genetically than brothers and sisters born at different times (physically dissimilar)
Always have 2 amnions and 2 chorions, but the chorions and placentas may be fused
They are different in blood characteristics and have no tolerance for transplants

MONOZYGOTIC (MONOVULAR AND MONOVITELLINE) TWINS result from fertilization of 1 oocyte. Occur in about 30% of twins and in 3 of 1000 births. They share fetal membranes almost completely

They are morphologically and physiologically identical and of the same sex
They are genetically identical and have identical blood characteristics
They have a tolerance for transplants
Any physical differences are due to environmental factors
Twinning usually begins at the end of week 1 and is the result of division of the inner cell mass into 2 embryonic primordia

There are 2 embryos, each in its own amniotic sac, which develop in one chorionic sac and have a common placenta
Early division of the blastomeres results in monozygotic twins with 2 amnions, 2 chorions, and 2 placentas. They cannot be typed by their membranes
Later division of the embryonic cells may result in monozygotic twins with 1 amnion and 1 chorion sa Such twins are rarely delivered alive because the cords are entangled and circulation stops, and 1 or both embryos die

CONJOINED TWINS: result of incomplete division of embryonic disk and are named by regions of attachment, e.g., thoracopagus (fusion of anterior thoracic region); pygopagus (fusion at back); or craniopagus or cephalopagus (fusion at head)

Triplets are seen in 1 of about 8000 pregnancies: may be identical (1 zygote); identical twins (2 zygotes) and a single infant; or 3 zygotes. May be of the same or different sexes
May see above in quadruplets (4); quintuplets (5); et These are rare but seen now as a result of treatment with gonadotropins for ovulatory failure
Superfetation: implantation of 1 or more blastocysts in a uterus that already has a developing embryo
Superfecundation: fertilization of 2 oocytes at the same time by sperm from different males (not proved in the human)