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:
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.
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 does 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.
These factors describe the mother. The father does not affect twinning.
Race: Blacks have a higher
rate of twinning than whites. Asians have the lowest rate.
Age: Twinning rates are highest between ages 35-39, after 40
the rate decreases except for black women, where the rate increases until age 49.
Sexual activity: People with higher sexual
activity have a higher rate of twins.
Height/weight: Taller and heavier women have a higher rate of twins.
children: Women with more children are more likely to have twins. A 4th or 5th pregnancy is four times more likely to be multiples.
Pill: Shortly after going off birth control, hormonal fluctations cause many women to ovulate twice.
twins, actually the inclination to ovulate more than once per cycle, seems to run in families, although no one has discovered
a gene (see below in Chances of Conceiving Multiples for more information). Twinning does not skip a generation, and just
because someone has fraternal twins in your family doesn't mean you will.
Fertility treatments: Hormonal treatments with
drugs and surgical reproductive treatments have dramatically affected the twinning rate.
There is nothing you can do
to increase your chances of having twins, and there is no clear way to predict twins, regardless of family history, fertility
CHANCES OF CONCEIVING MULTIPLES
Chances of identical twins: 1 in 285 (figure is universal,
identicals happen randomly)
Chances of fraternal twins: 1 in 43 (higher or lower in some countries - in Japan, it is 1
in 149 while in Nigeria it is 1 in 25)
Chances of conjoined twins: 1 in 50,000
Chances of more than one set:
sets of fraternal twins - 1 in 3000
1 set of identical/1 set of fraternal - 1 in 10,000
2 sets of identical twins -
1 in 70,000
Chances if YOU are a twin:
If you are fraternal: 1 in 17
If you are identical: does not affect chances,
so 1 in 38-90 (depending on if fertility treatments are used)
If your mother is a twin, you may have a higher than
average chance of twins. If your father is a twin and you are female, it may slightly increase your chances. Being married
to a twin does NOT affect your chances. If your parent is an identical twin, your chances of having twins are the same as
Chances of spontaneous triplets, in the US: 1 in 8100 for white women, 1 in 9800 for black women
Triplets and higher rate: 1 in 539
Fertility treatments have drastically affected the rate of higher-order (4+) multiples
being conceived. From 1900-1950, only 46 sets of quads were born worldwide. In 1994, at least 79 sets of quads were born just
in the US.
Average rates of higher-order multiples:
102 sets of quads are born every year in the US
12 sets of
quints are born every year in the US
1 set of sextuplets are born yearly in the US
1 in 5 Clomid pregnancies are
multiples. Clomid, an ovulation drug, will increase your chances of conceiving multiples by roughly 10%.
MULTIPLE GESTATION FACTS
2 = Twins
3 = Triplets
4 = Quadruplets
5 = Quintuplets
6 = Sextuplets
8 = Octuplets
9 = Nonuplets
10 = Decaplets
11 = Undecaplets
12 = Duodecaplets
13 = Tridecaplets
15 = Quindecaplets
Highest # of babies conceived spontaneously: A 24 year old Argentinan woman in
1992 miscarried duodecaplets, conceived without fertility intervention of any kind.
Highest # of babies born: Decaplets
(higher numbers have been conceived, but not carried long enough to be considered "born" rather than miscarried).
# of babies that all survived: Septuplets. There are two sets of septuplets in the world where all 7 babies survived, the
McCaugheys in the USA and Humair/bin Mohammed Sammam in Saudi Arabia. There is a set of octuplets in Texas where 7 survived.
higher order multiples born before the 1960s were more than likely conceived spontaneously.
There are 32 sets of sextuplets
(with all babies surviving) in the world. Most of them (12 sets) are in the US. The oldest of these sets was born in South
Africa in 1974, the youngest in Egypt in 2005. The first American set to survive was the Dilleys of Indiana in 1993. The first
African-American set to survive was the Harrises of Alabama in 2002. Longest gestation for sextuplets was 33 weeks (Martinez,
Highest number of identicals: Quintuplets. The Dionnes of Canada (5 girls, all monozygotic) were the
first set, in 1934, of identical higher-order multiples to survive. The girls were taken from their parents by the province
of Ontario and turned into a tourist attraction, living in a hospital with a viewing area much like a zoo. Interestingly,
one of the Dionne quints was left-handed, the other 4 girls were right-handed.
35% of twins are left-handed, twice the rate of non-twins.
Identical twins do not have the
Over 50% of twins are born before 37 weeks.
Children of identical twins are genetically half siblings.
identical twins marry identical twins, their children will legally be cousins, but genetically will be full siblings.
of twins are more likely to get divorced than parents of singletons.
If one half of a set of twins gets divorced, the other
twin is more likely to get divorced.
The divorce rate is higher among (adult) twins and multiples than among singletons.