How are triplets or sextuplets formed?

Triplets are three babies carried in the womb during a single pregnancy. 

Most triplets are trizygotic, meaning that each child forms from a separate zygote, or egg/sperm combination. They are commonly called 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.

Higher multiples like quadruplets, sextuplets etc. can often be a combination of identical and fraternal.