Multiple birth

A birth in which one pregnancy results in two or more offspring. The offspring may be twins, triplets, or a larger set.

Twin: One of two offspring produced by the same pregnancy. Twins are the most common form of multiple birth.
Fraternal twins: Twins that develop from two separate eggs fertilized by two separate sperm. They result when one pregnancy includes two independently fertilized eggs.
Prenatal care: Health care provided during pregnancy to monitor the pregnant person and developing fetus or fetuses. Multiple gestations require surveillance tailored to fetal number and placentation.
Maternal mortality: The death of a pregnant or recently pregnant person from causes related to pregnancy or its management. Multiple gestation is associated with greater maternal health risks than singleton pregnancy.
Triplet: One of three offspring produced by the same pregnancy. Triplets illustrate multiple births involving more than two offspring.
Identical twins: Twins that develop when one fertilized egg divides into two embryos. Embryo splitting produces two offspring from a single zygote.
Ultrasonography: Medical imaging that uses high-frequency sound waves to produce images of structures inside the body. Ultrasound identifies fetal number, chorionicity, growth, and position.
Low birth weight: A birth weight below 2,500 grams, regardless of gestational age. Multiples are more likely than singletons to be born with low birth weight.
Zygosity: The genetic relationship between siblings, especially whether they arose from one fertilized egg or separate eggs. Zygosity distinguishes identical from fraternal twins and shapes pregnancy risks.
Superfecundation: Fertilization of two or more eggs from separate acts of intercourse during one ovulatory cycle. It is one route to twins with different biological fathers.

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