Twins and multiples are classified by zygosity. Zygosity is a way to describe twins based on how they form.

Dizygotic—or fraternal—twins are the result of two separate sperm/egg combinations. They are essentially two siblings who are conceived at about the same time and share a womb. Dizygotic twinning can be hereditary or influenced by environmental factors such as maternal age, diet, or body type.

Monozygotic twins are commonly known as identical. Only about one-third of twins are monozygotic, forming from a single egg/sperm combination that splits and develops into two individual embryos. Because they share the same combination of DNA, they can be remarkably similar and may look alike. Monozygotic twins that separate late form conjoined twins.

Twin type can be difficult to determine. Sometimes zygosity can be confirmed before birth by using ultrasound to assess placental structure. However, DNA sampling will more conclusively confirm whether multiples are monozygotic or dizygotic.