Twin Pregnancy Information

TWINNING: CONCEPTION, GESTATION, BIRTH

Twins happen for two reasons: either two eggs are fertilized, or one egg is fertilized and then splits. A double-egg pregnancy is called dizygotic, or fraternal. Since there are two unrelated eggs fertilized by two unrelated sperm, fraternal twins are no more alike genetically than siblings from separate pregnancies. Dizygotic twins can be boy/boy, boy/girl, or girl/girl. A single-egg pregnancy is called monozygotic, or identical. Monozygotic twins have many variations in gestation but are genetically identical (same nuclear DNA) because they come from the same egg and same sperm. They will always be of the same sex. Monozygotic twins happen randomly, as a genetic mutation, and the cause of the zygote splitting is unknown. Depending on when the egg splits, monozygotic twins may or may not share a placenta and amniotic or chorionic sac. The later the egg splits, the more the twins will share during gestation. The earliest split would form dichorionic, diamniotic twins (two placentae/chorions, two amniotic sacs). A median split would form monochorionic, diamniotic twins (one placenta/chorion, two amniotic sacs within the single chorion). A later split would form what is known as mo-mo twins: monochorionic, monoamniotic (one placenta/chorion, one amniotic sac), which is the most dangerous gestation for twins as they can develop twin-to-twin transfusion syndrome or TTTS, which is potentially deadly. If the egg begins to split very late, after 12 days from conception, monozygotic twins will be conjoined twins - the division stops and they develop attached to each other. 60% of conjoined twins are stillborn or miscarried.

Odd/unusual twin formations:
Polar body twinning - also known as half-identical twinning, which has NOT been proven to occur in humans. The idea here is that an egg would split BEFORE fertilization, then both halves fertilized by separate sperm, resulting in 75% identical nuclear DNA. Again, it is UNPROVEN that this can or has occurred in humans or that it could produce viable fetuses.
Superfetation - This is when a woman conceives again while already carrying a fetus, when eggs are released at different times and fertilized as they are released. It can happen up to 24 days apart. Essentially, this is carrying two singleton pregnancies at the same time, with their own separate conception and due dates. This can also happen if a woman conceives via IVF and then conceives naturally shortly afterward. Superfetation is very rare.
Superfecundation - Sounds like an episode of the Maury show, but this happens when a woman ovulates twice in one cycle and then has more than one sex partner during her fertile period, resulting in fraternal twins with two different fathers. Genetically, these are half-siblings. This is very rare also.
Chimerism - This happens when one twin absorbs the other during gestation. It is only detectable if it happens with fraternal twins, resulting in one person with the DNA of two separate people, who may have two different blood types. This is extremely rare, only about 30 cases are known, but if identical twins were to merge it would be impossible to detect so the incidence of chimerism may be higher (but still very, very rare).
Turner's Syndrome (boy/girl identicals) - extremely rare genetic abnormality, where identical boys are conceived but one loses a Y chromosome during twinning (remember, male = XY, female = XX), resulting in a female with XO chromosomes who is genetically identical to her unaffected male twin. Turner's Syndrome is characterized by nucal folds, short height, space-form blindness, and abnormal secondary sexual development, and are usually infertile. This is an uncommon form of Turner's, which more commonly occurs in single births or even a fraternal birth, where a boy is conceived but loses the Y chromosome.

Commonality:
2 chorions, 2 amnions, 2 placentae (either fused or separate placentae): 1-4 days after conception, 25-30% of monozygotic gestations
1 chorion, 2 amnions, 1 placenta: 4-9 days after conception, 65-70% of MZ gestation
1 chorion, 1 amnion, 1 placenta: 9-12 days after conception, 1-5% of MZ gestation

Fused placentae do not necessarily indicate MZ twins: about 30% of cases of fused placentae are identical, and 70% are fraternal. Placental analysis is not conclusive in determining zygosity, as a fused placenta is difficult to distinguish from a single. The only sure way to determine zygosity is a DNA test.

Higher order multiples are formed similarly to twins. They may be monozygotic, and the egg split more than once (this is increasingly rare the higher gestation it is, i.e. identical trips are more common than identical quads, or identical quints), or dizygotic, where more than one egg is released and fertilized, or a combination of the two, for example, a set of quads where two are identical twins and the others are fraternal.

FACTORS AFFECTING TWINNING