Conjoined twins - Identical twins where separation is incomplete so their bodies are joined together at some point. Conjoined twins are sometimes referred to as "Siamese twins."

Diamniotic twins - Twins who have developed in separate amniotic sacs. These twins may be either dizygotic or monozygotic.

Dichorionic twins - Twins who have developed in separate chorionic sacs. These twins may be either dizygotic or monozygotic.

Dizygotic (dizygous) twins - Twins formed from two separate zygotes. Dizygotic twins are commonly referred to as "fraternal twins."

Embryo reduction - See fetal reduction.

Fetal reduction - See Multifetal Pregnancy Reduction

Feto-fetal transfusion syndrome - See Twin to Twin Transfusion Syndrome (TTTS).

Fetus papyraceous - A fetus that dies in the second three months of pregnancy and becomes compressed and parchment-like.

Fraternal twins - See Dizygotic twins.

Higher-order multiples - Triplets, quadruplets, quintuplets or more.

Higher-order pregnancy - A pregnancy with three or more fetuses.

Identical twins - See Monozygotic twins.

Infant death - Death in the first year of life.

Monoamniotic twins - Twins who have developed in a single amniotic sac. These twins are always monozygotic.

Monochorionic twins - Twins who have developed in a single chorionic sac. These twins are always monozygotic.

Monozygotic (monozygous) twins - Twins formed from a single zygote. Monozygotic twins are commonly referred to as "identical twins."

Multifetal Pregnancy Reduction - The reduction in number of viable fetuses/embryos in a multiple (usually higher multiple) pregnancy by medical intervention.

Multiple pregnancy - A pregnancy with more than one fetus.

Neonatal death - A death in the first four weeks after delivery.

Perinatal mortality - A stillbirth or death during the first week after delivery.

Selective fetocide - The medical destruction of one or more fetuses in a continuing pregnancy.

Selective reduction - See Fetal reduction.

Siamese twins - See conjoined twins.

Stillbirth - A baby born at 24 weeks gestation or later who shows no signs of life.

Superfecundation - Conception of twins as a result of two acts of sexual intercourse in the same menstrual cycle.

Superfetation - Conception of twins as a result of two acts of sexual intercourse in different menstrual cycles.

Supertwins - Higher order multiples: triplets, quadruplets, quintuplets or more.

Trizygotic - Formed from three separate zygotes.

Twin-to-twin transfusion syndrome (TTTS) - A condition in which blood from one monozygotic twin fetus transfuses into the other fetus via blood vessels in the placenta. This condition can also occur among monozygotic multiples in a higher-order multiple pregnancy.

Vanishing twin syndrome - The reabsorption of one multiple fetus early in the pregnancy, usually by the twelfth gestational week.

Zygosity - The genetic relationship between or among children from a multiple birth (i.e., whether the children are monozygotic or dizygotic multiples).

Zygosity testing - Testing done to determine the zygosity of children from a multiple birth. The only completely reliable form of zygosity testing is DNA fingerprinting.

Zygote - A fertilized ovum.