Multiple gestations are rare but now occur at a greater frequency than in the past due to the effects of infertility treatment. While previously a frequency of 1 out of 85 was observed, today 1 in 40 births worldwide involves twins. Twinning associated with infertility treatment is called dizygotic. In the absence of ovulation enhancement therapy, approximately two-thirds of twin pregnancies are dizygotic. Factors associated with dizygotic pregnancies include the ethnic background of the mother, maternal age, reproductive medicine techniques, and as yet undetermined genetic factors (i.e., “Twin pregnancies run in our family”). In contrast, there are no known associated “risk factors” for monozygotic twinning.
Monozygotic twins (MTs):
Twins that result from the fertilization of one ovum by one sperm and the subsequent division into two zygotes.
Dizygotic twins (DTs):
Twins that result from simultaneous yet independent maturation, ovulation, and fertilization of two ova.
It is obvious that dizygotic twins share no more genetic information than siblings, whereas monozygotic twins are expected to have the same DNA sequence. However, it is not always clear during a gestation or even at delivery if the twins are monozygotic or dizygotic. If one twin is male and one is female, they are almost certainly dizygotic; rare exceptions exist and involve sex chromosomal abnormalities.
The examination of the placenta and fetal membranes is only helpful in determining zygosity when a monochorionic placenta is observed, as only monozygotic twins will have this type of placentation. Any other type of placentation (e.g., two completely separate placentas or a single [fused] placental disc with two separate amniotic sacs) can be observed in either monozygotic or dizygotic pregnancies. In those cases DNA analysis is used to determine zygosity, as physical appearance of infants and even small children can be deceiving.
Concordance and Discordance
Studying phenotypical characteristics of MTs and DTs can, under certain conditions, provide information about which part of the phenotype is genetically determined. If twins have the same phenotypical characteristic, they are concordant; if they differ, they are discordant with regard to this characteristic. If a genetic disorder is determined mainly through genetic factors, concordance will be much higher for MTs than for DTs. In an extreme case, all MTs would be concordant, while the concordance rate of DTs would be the same as that of siblings. If, however, external factors play a major role, the phenotype is independent of the twin status, and there will only be a slight difference in concordance rates between MTs and DTs.
Placental Morphology in Twin Pregnancies
The typing of a twin placenta is done macroscopically as well as histologically. Classification occurs according to the number of chorions and the number of amnions (Fig. 6.1).
Figure 6.1.The Morphology of Twin Placentas.
The multilayered, outermost fetal membrane, consisting of extraembryonic somatic mesoderm, trophoblast, and, on the maternal side, villi bathed by maternal blood (chorion [Gr.]). Part of the chorion becomes the fetal placenta.
The thin, avascular inner membrane that envelops the embryo in utero (amnion [Gr.], “sheepskin,” “skin surrounding the fetus”). Together with the embryonic ectoderm, it forms the amniotic sac.
Early embryonic division (within the first 3 days after fertilization) results in a monozygotic twin pregnancy with dichorionic, diamniotic placentation. This scenario occurs in approximately 25% of all MTs. Division within 4 to 8 days after conception results in a monochorionic diamniotic placenta (74% of all MTs); division occurring between 8 and 12 days after fertilization results in a monochorionic monoamniotic placenta (less than 1% of all MTs).
All monochorionic placentas can be traced back to a monozygotic twin pregnancy; dizygotic twins never share a placenta. However, an important caveat to this rule is evidenced when placental discs are fused and appear to be monochorionic.
Placental Morphology in Twin Pregnancies
- Monozygotic twins often have a common placenta; dizygotic twins almost never share a common placenta (except in the case of a fused placenta).
- Only monozygotic twins share a common amniotic sac.
- Monozygotic twin pregnancy can never be excluded by placental morphology, while dizygotic twin pregnancy can be excluded in all truly monochorionic placentas.
Twin–Twin Transfusion Syndrome
Twin–twin transfusion syndrome (TTTS) is a serious complication of monochorionic twin pregnancies, where one fetus transfuses blood into the other twin. This is caused by placental shunts, especially arteriovenous anastomoses. If TTTS occurs as early as the second trimester, the prognosis is poor. The mortality rate without therapy is 80% to 100% and is caused by anemia and intrauterine growth retardation in the donor and cardiovascular decompensation in the acceptor.
Conjoined twins (sometimes called “Siamese” twins in reference to Chang and Eng Bunker, who were born on May 11, 1811, in Siam [now Thailand]) are monozygotic twins whose failed separation results from the twinning process initiating approximately 13 to 14 days after fertilization. Conjoined twins have exclusively monochorionic/monoamniotic placentation; however, the twins themselves can be joined at the head (cephalopagus), chest (thoracopagus), abdomen (omphalopagus), sacrum (pygopagus), pelvis (ischiopagus), or xiphoid process of the sternum (xiphopagus). The incidence of conjoined twins is approx 1:100,000 births. There are many accounts of conjoined twins either living in a conjoined state or being surgically separated.
Besides conjoined, fully developed twins, there are also cases of asymmetric twinning, where one of the twins is incompletely developed. A conjoined twin that is small, poorly formed, and dependent on the larger twin for survival is called a “parasitic” twin. Parasitic twins are attached at various regions of the body (e.g., highly differentiated dermoid cyst, sacral parasite) and can be removed surgically.