Formation of Twins. Intermediate

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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.