monozygotic

Monozygotic refers to a developing organism that arises from the division of a single fertilized egg (zygote) into two embryos. In humans, monozygotic development normally results in identical twins, who share the same genetic material.

The way the zygote splits determines the placentation and amnionicity of the twins. If the split occurs very early, within about the first four days after fertilization, two separate placentas and two amniotic sacs form (dichorionic diamniotic). If the split happens a little later, around days 4 to 8, the twins typically share one placenta but have separate amniotic sacs (monochorionic diamniotic). Splitting around days 8 to 12 usually yields one placenta and one amniotic sac (monochorionic monoamniotic). Very late splits, after about day 13, can result in conjoined twins, which share body regions due to incomplete separation.

Incidence of monozygotic twinning is about 3 to 4 per 1,000 live births, making it less common than dizygotic twinning. The condition is not linked to maternal age or ethnicity, though the overall rate can be influenced by assisted reproductive technologies.

Genetically, monozygotic twins are nearly identical, having the same DNA sequence. Post-zygotic mutations and epigenetic differences can introduce small variations between them, and their experiences in the womb can lead to differences in development and health outcomes.

Clinical considerations vary by placentation type. Monochorionic pregnancies carry higher risks, such as twin-to-twin transfusion syndrome and growth discordance, requiring careful prenatal monitoring and, in some cases, fetal therapy.