dizygotic

Dizygotic, often shortened to DZ, refers to twins that originate from two separate zygotes. The term derives from Greek elements meaning “two zygotes.” DZ twins arise when two ova are fertilized by two separate sperm cells, producing two genetically distinct individuals.

Genetically, dizygotic twins are as related as regular siblings, sharing about 50 percent of their DNA on average. They can be the same sex or different sexes and may resemble each other closely or appear quite different. DZ twinning is distinct from monozygotic (identical) twinning, in which a single zygote splits to form two genetically identical individuals.

Development and placentation are typically dichorionic diamniotic, meaning each twin generally has its own placenta and amniotic sac, though placentas can sometimes appear fused. This is in contrast to most monozygotic twins, whose placentation patterns depend on the timing of the zygote’s split.

Incidence and risk factors for dizygotic twinning vary by population and maternal characteristics. DZ twinning is more common with increasing maternal age, higher parity, and a family history of fraternal twinning on the maternal side. Use of assisted reproductive technologies and certain fertility treatments also raises the likelihood. Environmental and genetic factors contribute to the probability but do not guarantee twinning.

Diagnosis is typically made by prenatal ultrasound, which can reveal two gestational sacs and two fetuses. Postnatal examination or genetic testing can confirm dizygosity. Dizygotic twinning carries higher risks for twin pregnancies in general, such as preterm birth and low birth weight, requiring careful prenatal monitoring.