Formation of Twins: Intermediate
Ebix Inc.
Twins are rare, occurring in about 1 percent of all pregnancies. Of that number, 30 percent are identical twins. The other 70 percent are nonidentical, or fraternal, twins. A single baby, identical twins, and fraternal twins differ from one another in their development. With a single baby, when fertilization occurs the egg cell is fertilized by a single sperm cell to form a zygote. Over the next few days, the fertilized egg cell divides repeatedly to form a structure called a blastocyst, which is composed of hundreds of cells. If you looked inside the one-week-old blastocyst, you would see a mass of cells that will form the embryo. The blastocyst continues traveling toward the uterus, where it will implant in the uterine lining and grow into a single baby. Identical twins start from a single fertilized egg cell, or zygote, and therefore are called monozygotic twins. The egg cell is fertilized by a single sperm cell, as it is with a single baby. Unlike the zygote of the single baby, this fertilized egg cell will split into two separate embryos and grow into identical twins. The remarkable splitting event takes place during the first week after fertilization. It can happen at several different times: at the two-cell stage on day 2, at the early blastocyst stage on day 4, or in the late blastocyst stage on day 6. The stage at which the zygote splits determines how the twins will implant in the uterine lining and whether they share an amnion, chorion, and placenta. The earlier the splitting occurs, the more independently the twins will develop in the uterus. Identical twins that split during the two-cell stage will each develop a separate amnion, chorion, and placenta, while twins that split during the late blastocyst stage will share an amnion, chorion, and placenta.A common misconception about identical twins is that the trait for having them is passed on to future generations through the mother’s genes. The truth is that science does not know why identical twins occur. At this time, it can just be said that identical twins are examples of a nine-month double miracle. Nonidentical, or fraternal, twins develop from two fertilized egg cells, or zygotes, which is why fraternal twins are also called dizygotic twins. While a genetic tendency for identical twinning is unknown, the likelihood of bearing fraternal twins is definitely influenced by the mother’s genes. Normally a woman’s ovaries release only one egg during ovulation. When a mother of fraternal twins ovulates, sometimes her ovaries release two egg cells. During conception these egg cells become fertilized by two different sperm cells. Because they are the products of different egg cells and different sperm cells, fraternal twins do not look exactly alike. Sometimes they are not even the same sex. In the uterus fraternal twin embryos develop separately, each one having his or her own chorion, amnion, and placenta.