Multiple pregnancies seem to be the topic of the day. Twins represent the majority of multiple pregnancies. The chance of having twins naturally is approximately 1%. Most twins are fraternal (non-identical) meaning that they each come from one egg. Fraternal twinning appears to have a genetic basis since twins tend to occur repetitively in certain families and have increased incidences in certain ethnic groups. Naturally occurring fraternal twins occur when a woman ovulates at least two eggs and both are fertilized. Usually, women ovulate one egg each month.

Identical twins are the result of a single embryo splitting into two embryos during the first 15 days of life. Identical twinning is thought to represent a random event rather than having a genetic or racial basis.