Triplets are three babies carried in the womb during a single pregnancy. Like twins, triplets and other higher order multiples can be categorized by their zygosity.
Most triplets are trizygotic, meaning that each child forms from a separate zygote, or egg/sperm combination. They are commonly described as "fraternal" mulitples, and share the same genetic similarities as any siblings. However, it is not uncommon for triplets to be dizygotic, which occurs when two eggs are fertilized by sperm, and one of the fertilized eggs splits into two. Essentially, two of the triplets are monozygotic -- or identical -- twins, sharing the same general DNA characteristics, while the third multiple has a unique genetic heritage.
It is rare for triplets to be completely monozygotic, meaning that all three children formed from a single egg that split three ways, or split into two and then one of the two split again, ultimately resulting in three embryos with the same general DNA characteristics. Some monozygotic triplets are actually quadruplets where one embryo has vanished or been reabsorbed.
Monozygotic triplets are always of the same gender, either all boys or all girls.
Recently, identical triplets made headlines when a monozygotic triplet pregnancy resulted after a single embryo was implanted in an in-vitro procedure. Allison Penn gave birth to Logan, Eli and Collin in March 2008. Her doctors identified the situation as the only known case of a single embryo transfer resulting in triplets.
What are the Odds?
Research varies on the instances of monozygotic triplets. Estimates range between one in 60,000 and one in 200 million. When a British woman gave birth to identical girls in Austria in August 2007 without fertility interventions, the event was termed as something that only happens "in about one in 200,000 pregnancies", or in one out of every six triplet pregnancies. The article estimated that about ten sets of identical triplets are born in Britain each year.
In 2013, a set of identical girls were born in California. Dr. William Gilbert, director of Women's Services for Sutter Health in Sacarmento, explained that the occurrence is so rare that it was difficult to accurately measure statistically. He placed the odds at one in 1 million to one in 100 million.
There doesn't seem to be a clear definition of what exactly the odds are, perhaps because it is rare. Another explanation of the variance may be in defining triplet conceptions/pregnancies and live triplet births. A monozygotic triplet pregnancy carries the risks of a monozygotic pregnancy with a shared placenta, and the additional risks of a triplet pregnancy. A pregnancy of this type is more likely to encounter problems such as pre-eclampsia, preterm labor, and even Transfusion Syndrome.