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MECHANISMS OF MULTIFETAL GESTATIONS ......... 864
DIAGNOSIS OF MULTIFETAL GESTATION ........... 869 MATERNAL PHYSIOLOGICAL ADAPTATIONS ........ 870
PREGNANCY COMPLICATIONS ....................i. 871
UNIQUE FETAL COMPLICATIONS ................... 873 DISCORDANT GROWTH OF TWIN FETUSES .......... 881
FETAL DEMISE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 882 PRENATAL CARE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 884 PRETERM BIRTH ................................ 885
LABOR AND DELIVERY ......................... 887
SELECTIVE REDUCTION OR TERMINATION .......... 891
In single-ovum twins, there is always a certain area of the placenta in which there is anastomosis between vascular systems which is never present in the fused placenta of doubleovum twins. Thus, if at an eary period the heart of one embryo is consideraby stronger than that of the other, a gradualy increasing area of the communicating portion of the placenta is monopolized by the ormer, so that its heart increases rapidy in size, whilst that of the latter receives less blood and eventualy atrophies.
-J. Whitridge Williams (1903)
In Williams' time, a great deal concerning the embryological and morphological development of multifetal pregnancies was unknown. These pregnancies may result from two or more fer tilization events, from a single fertilization followed by a split ting of the zygote, or from a combination of both. Multifetal today for both the mother and her fetuses. For example, in this country, approximately a fourth of very-low-birthweight gestations (Martin, 2017).
Fueled largely by infertility therapy, both the rate and the number of twins and higher-order multi fetal births grew dramatically during the 1980s and 1990s in the United States. National data from Martin and coworkers (2017) presented here is informative. he twinning rate rose 76 percent from 18.9 per 1000 live births in 1980 to 33.2 in 2009. During the same time, the number of higher-order multifetal births peaked in 1998 at a rate of 1.9 per 1000 total births. Since then, however, evolving infertility management has lowered rates of higher-order multifetal births-especially among nonHispanic white women. For example, the rate of triplets or more declined by more than 50 percent from 1998 to 2015 in this demographic group. And, in 2015, the overall m ultifetal birth rate was 34.5 per 1000, with twins representing nearly 97 percent of these births.