23/11/2025
What they are: Identical twins develop from a single fertilized egg (zygote) that splits into two embryos. Because they share the same original DNA, they are genetically very similar, typically calling for a common genome for both individuals.
How the split occurs: The timing of the split in early embryonic development largely determines placental and amniotic configurations:
Very early split (within the first 0–4 days): usually results in dichorionic diamniotic twins (two placentas, two amniotic sacs) similar to fraternal twins in placental structure, though still monozygotic.
Split around day 4–8: often monochorionic diamniotic (one placenta shared, but with two amniotic sacs).
Split after day 8–12: can be monochorionic monoamniotic (one placenta and one amniotic sac), with higher risk features.
Very late splits are rare and can be associated with conjoined twins.
Genetic makeup: They arise from one zygote and start with nearly identical genomes. However, they are not absolutely identical:
Post-zygotic somatic mutations can create genetic differences between twins.
Epigenetic differences (differences in DNA methylation, histone modification, etc.) can alter gene expression between twins.
Random X-chromosome inactivation in females can lead to differences in which X is active in different tissues, influencing phenotype.
Environmental factors in utero (e.g., nutrient sharing, placental blood flow) can lead to subtle differences.
Physical and developmental similarities/differences:
They usually look alike and often have similar facial features, voice timbre, and handwriting, but are not identical in appearance or personality.
Fingerprints are not identical, though they may be very similar at a high level; they are uniquely formed by random developmental processes.
They can have different birth weights, growth rates, and medical histories.
Health considerations and risks:
Chorionicity and amnionicity matter for pregnancy risk. Monochorionic twins (one placenta) have higher risk for twin-to-twin transfusion syndrome (TTTS) and other placental sharing issues.