The “cell cluster” is the early embryo, which transitions from a zygote into a blastocyst within the first week after fertilization. The splitting of this cluster is the mechanism that leads to the formation of identical twins. This spontaneous event results in two genetically identical individuals who developed from that one initial fertilized egg.
The Process of Monozygotic Twinning
Monozygotic, or identical, twinning begins with a single fertilization event, creating one zygote. This cell divides, forming a cluster of cells known as the morula, which subsequently develops into the blastocyst. The blastocyst is structured into two main parts: the inner cell mass (ICM) and the trophoblast. The ICM is destined to become the fetus, while the trophoblast forms the outer membranes and the placenta.
Identical twinning occurs when the ICM splits into two separate groups of cells. Each new group continues to develop independently, effectively forming two embryos. Since both embryos originated from the same fertilized egg, they share nearly identical genetic material. The exact cause for this splitting remains largely unknown, but it can happen at various stages of the embryo’s early development.
How Timing Affects Twin Development
The specific structures that the twins will share during the pregnancy depend entirely on when the splitting of the inner cell mass occurs. The outer membrane, which gives rise to the placenta, is called the chorion, and the inner fluid-filled sac is the amnion.
If the split happens very early, between Day 1 and Day 3, the result is dichorionic-diamniotic (Di/Di) twins. Each embryo develops its own separate chorion and amnion, meaning they have individual placentas and separate sacs.
A slightly later split, occurring between Day 4 and Day 8, results in monochorionic-diamniotic (Mono/Di) twins. These twins share one placenta, derived from a single chorion, but they still maintain separate inner amniotic sacs.
A split that occurs even later, typically after Day 9, leads to monochorionic-monoamniotic (Mono/Mono) twins. In this instance, the twins share both a single placenta and a single amniotic sac, a situation that carries the highest risks for the pregnancy. The majority of identical twins, about 75%, fall into the Mono/Di category, sharing a single placenta.
When Splitting is Incomplete
The latest possible timing for the cell cluster to split marks the point where complete separation may not occur. If the division of the embryonic disc is delayed until Day 12 or later, the process can be incomplete, resulting in conjoined twins. This rare outcome is a direct consequence of the embryo attempting to divide after the foundational body plan has already been established.
The location where the split fails to complete determines which body parts and organs the twins will share. Conjoined twins are always monoamniotic. They are classified based on the point of connection, such as being joined at the chest or abdomen. The challenge for these twins often lies in the degree of shared organs, particularly vital ones like the heart or liver.
Distinguishing Identical from Fraternal Twins
The splitting of the cell cluster is the defining event for identical, or monozygotic, twins. They arise from one single egg fertilized by one sperm, which then divides to create two embryos with virtually the same genetic code. Consequently, identical twins are always the same sex and possess a striking physical resemblance.
In contrast, fraternal, or dizygotic, twins form through a completely different biological process. This occurs when the mother releases two separate eggs during a single cycle, and each of those eggs is fertilized by a different sperm cell. These two separate fertilization events mean that fraternal twins are genetically distinct, sharing only about half of their DNA, just like any other siblings born years apart. Fraternal twins may or may not be the same sex and do not necessarily resemble each other closely.