National Twin Day

Most people know someone who is a twin. Perhaps they are a family member or friend. Maybe they are a neighbor or an acquaintance from work or school. If they don't know any twins firsthand, they are bound to be familiar with famous or celebrity twins. National Twin Day was proclaimed "to celebrate all things twins," and is a day that "celebrates all the siblings who enjoy the unique connection because they share a birthday."

About two out of three twins are fraternal, or dizygotic, coming from two zygotes (fertilized eggs), and the other third are identical, or monozygotic, coming from one zygote. Fraternal twins develop when two eggs are released from a woman's ovaries and are fertilized by two sperm. The two fertilized eggs produce two children who are genetically unique. The appearance of the twins may be quite similar or may be different. During gestation, fraternal twins receive food and oxygen through separate placentas and umbilical cords.

Identical twins develop when one fertilized egg splits or divides in two within a few days of fertilization. Two babies with the same genetic information develop. They look almost identical, but there may be some differences in appearance due to different experiences in the womb and after birth. Unlike fraternal twins, identical twins are always the same sex.

There are three types of identical twins. About a third come from zygotes that split right after fertilization. These twins have separate placentas. Most of the other two-thirds of twins come from zygotes that split after attaching to the wall of the womb. These twins share placentas. A small number of zygotes split later than this. These twins not only share a placenta, but the inner sac called the amnion. Those sharing a placenta are more prone to complications, and those sharing amniotic fluid are even more prone. When they share a placenta they share a blood supply, which each twin might not receive equally. When they share an amnion, their umbilical cords may get wrapped together and cut off nutrients.

Factors increase the likelihood that a woman will have twins. She is more likely to have them if she is in her 30s or 40s because older women have higher levels of the sex hormone estrogen than younger women, which makes the chances higher that they will produce more than one egg at a time. The more previous pregnancies a woman has had, the higher odds that she will have twins. The odds also go up if a woman is a fraternal twin, has had fraternal twins, or has siblings who are fraternal twins. Race can play a factor: Blacks have twins at the highest rate, while Asians have them at the lowest. Assisted reproductive techniques may also play a role: ovaries may be stimulated, leading them to release more than one egg during ovulation.

How to Observe National Twin Day

Spend the day celebrating twins! Give a shout-out to friends or family members who are twins, or to some famous twins. If you happen to be a twin, share photos and memories, or host a twin party where you invite all the twins you know. Post about the day on social media with the hashtag #NationalTwinDay.

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