How Do Multiple Births Occur?
Multiple pregnancy cases that cause complications are quite common, and there are many dramatic examples of children who do not reach adulthood. However, the story of the McCaughey family of seven is a beautiful testimony of what many call a miracle.
This undated screen grab shows the cell-division of two fertilized human embryos during the first 24 hours of embryonic development following IVF treatment at a private clinic in London. (Photo by Jim Dyson/Getty Images)
Multiple births are not as uncommon in the modern world as they once were, mostly because of the innovations in fertility treatments. Twins are, of course, the most common occurrences, with approximately 1 in 30 US births being a double one. Next come triplets, with women having a 0.1% chance of having them. As the number of babies grows, the chances of having them get smaller. For example, there is a 0.0078% chance of having four babies (quadruplets) and a 0.00092% chance of having five babies (quintuplets) or more.
When an egg either separates into two or more embryos or more than one egg is fertilized, multiple births happen. If an egg splits into two or more embryos, the twins will share the same genetic material, which makes them identical. On the other hand, if more than one egg is fertilized in the womb, the fraternal fetuses will share just 50% of their genetic material. They will be the same as siblings, just born at the same time.