One embryo can split to form twins. This results in identical, or monozygotic, twins. The splitting of a single fertilized egg leads to individuals who share remarkable similarities. This process contrasts significantly with other types of twinning, which arise from different biological pathways.
Understanding Identical Twins
Identical twins, or monozygotic twins, originate from a single fertilized egg, or zygote. This zygote divides into two separate embryos early in development. Because they develop from the same initial genetic material, identical twins share nearly identical DNA. This shared genetic blueprint contributes to their resemblances in physical characteristics.
Identical twins are always of the same sex, as they derive from the same combination of sex chromosomes present in the original zygote. Subtle differences can emerge due to environmental factors, both during prenatal development and throughout their lives.
The Process of Embryo Splitting
The formation of identical twins begins after fertilization, when a single sperm fuses with a single egg to create a zygote. This zygote undergoes rapid cell divisions, a process known as cleavage. For identical twins to develop, this early embryo undergoes a division, forming two distinct embryonic structures.
The exact biological mechanisms that trigger this spontaneous splitting are not fully understood, but it happens during the initial stages of embryonic growth. This division typically occurs before the embryo implants in the uterine wall. The inner cell mass, or the entire early embryo, separates into two complete entities, each capable of developing into a fetus.
Variations in Splitting
The timing of the embryo’s division influences how the identical twins will share their gestational sacs and placentas. If the split occurs early, within the first three days after fertilization, the twins develop with separate chorions and amniotic sacs, and often separate placentas. This type, dichorionic-diamniotic (DiDi), accounts for about 25% of monozygotic twin pregnancies.
Splitting between days four and eight post-fertilization typically results in twins who share a single chorion and placenta but have separate amniotic sacs. These are called monochorionic-diamniotic (MoDi) twins and represent a majority of identical twin pregnancies. A later division, occurring between days nine and twelve, leads to monochorionic-monoamniotic (MoMo) twins, where both the chorion and the amniotic sac are shared. This is the least common type of identical twinning, occurring in 1-2% of monozygotic twin pregnancies.
In rare instances, if the embryonic division occurs after approximately 13 to 15 days, the separation may be incomplete, leading to conjoined twins. These twins remain physically connected, often sharing organs or body parts, with the specific point of connection depending on the extent of the incomplete separation.
Distinguishing from Fraternal Twins
Understanding the process of embryo splitting is clarified by contrasting it with the formation of fraternal twins. Fraternal twins, also called dizygotic twins, result from a different biological event. Instead of one embryo splitting, fraternal twins develop when two separate eggs are fertilized by two distinct sperm cells during the same ovulatory cycle.
Because they originate from two different fertilization events, fraternal twins are genetically no more alike than any other siblings born at separate times. They each have their own unique genetic makeup, typically sharing about 50% of their DNA, similar to non-twin siblings. Fraternal twins can be of the same sex or different sexes, and they always develop with their own separate placentas and amniotic sacs.