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Alan H. Klein
Jill Alison Ganon
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What Are Twins?Fraternal and Identical TwinningThe miracle of conception takes place when an egg is fertilized by a sperm to become an embryo. Twins are defined as the simultaneous development of two embryos in a woman's uterus. After fertilization takes place, the fertilized ova enters the uterine cavity and implantation occurs. This phenomenon results in early differentiation of cells in each embryo: some develop into the extraordinary organ we know as the placenta, and other cells form the membranous, two layered sacs that will house each embryo. (On rare occasions, both embryos will reside in one sac.) The outer layer of the membrane is the chorion, and the inner layer is the amnion. Sounds pretty straightforward, right? But this is just the beginning. This book will talk about the two types of twinning: fraternal and identical.Scientists refer to fraternal twins as dizygotic, the union of two eggs with two sperm to create two embryos. Dizygotic twins are by far the most common form of twinning, and are two completely separate individuals who are as genetically similar as any siblings born at any time to the same parents.Identical, or monozygotic, twins are the result of one egg that has been fertilized by one sperm. At some point in the very early stages of cell division, the developing cell group splits in two and develops as two separate embryos. This rare and remarkable twinning creates two embryos who share the complete complement of each other's genetic information; their DNA is identical. The point at which the split occurs will determine which of four possible developmental scenarios will develop for the babies.This is a good time to talkabout the difficulties in diagnosing identical twins. Contrary to what you may have heard, it is possible for two embryos to appear to be completely independent of each other within the uterus and still be genetically identical. In the majority of twin pregnancies, the embryos develop with a minimal amount of shared placenta, and that is the best scenario you and your practitioner can hope for. The placenta is magnificently suited to supply your twins with everything they need throughout their gestation. Under the best of circumstances it generously provides for the transfer of oxygen, water, and nutrition from the mother to each of her developing fetuses. There are rare situations in which the placenta appears to provide a preferential supply of nutrition to one fetus over the other. This can result in discordancy and while it may be very subtle, it can often be identified early, allowing practitioners to optimize outcomes for both babies.It is interesting to note that the prevalence of twins occurring at conception is substantially higher than the birth rate would indicate, but due to a phenomenon called vanishing twin syndrome, one of the embryos may fail to develop and be resorbed by the body even before the diagnosis of pregnancy is made. We'll discuss that in more detail later on in this book. The Factors That Influence TwinningThe first thing that captures our interest when we discuss twins is the incidence of twinning itself. Naturally, when we look at birth rates, the occurrence of twinning falls within a limited range. But if you look at the most recent data in the medical journals, the frequency of fraternal twinning in the overall population is about one in fiftylive births. Interestingly, identical twins make their rare and thrilling appearance without regard to any of the factors that impact the incidence of their fraternal counterparts. Identical twinning occurs quite randomly throughout the population, but we are able to isolate and examine several variables that factor into the likelihood of a couple having fraternal twins: RaceMany people are amazed to learn that there is a racial component to the incidence of twinning in the world population. Twins occur in decreasing numbers within the Black, Caucasian, Hispanic, and Asian populations, respectively. And even within that demographic breakdown, the incidence of twinning among the Yoruba tribe of Western Nigeria is a stunning forty-five per thousand live births. This represents a rate of twinning that is four times that of the overall population When we measure the naturally occurring hormone levels present in the Yoruba women we find levels that are equal to those in women receiving clomiphene citrate (Clomid) to stimulate their ovaries to produce multiple eggs. We have not yet seen a study that definitively explains the extraordinary fertility of Yoruba women, but speculation ranges from genetics to nutrition to environmental considerations. Mother's Age and Obstetric HistoryWe also know that maternal age factors into the incidence of twinning. Mother Nature, with her droll sense of humor, visits twins upon mothers of increasing age and, as if that were not enough, the forty-year-old mom who already has children is more likely to conceive twins than her forty-year-old neighbor who has never had children. So far, the scientific community is at a loss to explain this. Weknow that in order to conceive fraternal twins, the mother must release more than one egg at a time. Yet we do not have data that absolutely suggest that a woman releases more eggs per cycle as she gets older. This increased rate of twinning for older moms does seem to drop off after the early forties, showing Mother Nature's ability to temper her humor with a little common sense. GeneticsThe role of genetics in twinning remains open to continued investigation. It is commonly accepted that there may be an inherited trait on the maternal side that contributes to an increase in twin births, though a mother who is a twin has no guarantee that she will give birth to twins. There is, however, no equally compelling data to suggest a similar influence may come from the father. Infertility TreatmentsThe final factor known to influence twinning
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