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FREQUENCY OF TWINS.

The frequency of monozygotic twin births is relatively constant worldwide, at approximately one set per 250 births, and is largely independent of race, heredity, age, and parity.

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If division occurs before the inner cell mass (morula) is formed and the outer layer of blastocyst is not yet committed to become chorion, that is, within the first 72 hours after fertilization, two embryos, two amnions, and two chorions will develop There will evolve a diamonionic, dichorionic, and monozygotic twin pregnancy. There may be two distinct placentas or a single fused placenta.

• If division occurs between the fourth and eighth day, after the inner cell mass is formed and cells destined to become chorion have already differentiated but those of the amnion have not, two embryos will develop, each in separate amnionic sacs. The two amnionic sacs will eventually be covered by a common chorion, thus giving rise to diamnionic, monochorionic, monozygotic twin pregnancy

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If, however, the amnion has already become established, which occurs about 8 days after fertilization, division will result in two embryos within a common amnionic sac, or a monoamnionic, monochorionic, monozygotic twin pregnancy.

• If division is initiated even later, that is after the embryonic disk is formed, cleavage is incomplete and conjoined twins are formed.

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ETIOLOGY OF MULTIPLE FETUSES

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HEREDITY.

As a determinant of twinning, the family history of the mother is much more important than that of the father.

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MATERNAL AGE AND PARITY.

The rate of twinning rises from 0 at puberty, a time of minimal ovarian activity, to a peak at age 37, when maximal hormonal stimulation increases the rate of double ovulation

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DIAGNOSIS OF MULTIPLE FETUSES

ULTRASOUND

By careful ultrasonic examination, separate gestational sacs can be identified very early in twin pregnancy. Subsequently, the identification of each fetal head should be made in two perpendicular planes so as not to mistake a cross-section of the fetal trunk for a second fetal head. Ideally, two fetal heads or two abdomens should be seen in the same plane, to avoid scanning the same fetus twice and interpreting it as twins. Sonographic scanning should detect practically all sets of twins.

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HISTORY AND CLINICAL EXAMINATION

A maternal family history of twins, older maternal age, high parity, large maternal size, and a previous history of twins provide weak clues, but knowledge of recent administration of either clomiphene or gonadotropins or pregnancy accomplished by assisted reproductive technology provide strong ones.

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In the case of a woman with a uterus that appears large for gestational age, the following possibilities are considered:

1. Multiple fetuses.

2. Elevation of the uterus by a distended bladder.

3. Inaccurate menstrual history.

4. Hydramnios.

5. Hydatidiform mole.

6. Uterine myomas.

7. A closely attached adnexal mass.

8. Fetal macrosomia late in pregnancy.

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OTHER DIAGNOSTIC AIDS

FETAL PARTS.
When uterine palpation leads to diagnosis of twins, it is most often because two fetal heads have been detected, often in different uterine quadrants.
In general, however, it is difficult to diagnose twins by palpation of fetal parts before the third trimester. Even late in pregnancy it may be very difficult to identify twins by transabdominal palpation, especially if one twin overlies the other, if the woman is obese, or if hydramnios is present.

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FETAL HEART SOUNDS.

Late in the first trimester, fetal heart action may be detected with generally available Doppler ultrasonic equipment. Sometime thereafter it becomes possible to identify two fetal hearts if their rates are clearly distinct from each other as well as from that of the mother. It is possible by careful examination to identify fetal heart sounds in twins with the usual aural fetal stethoscopes at 18 to 20 weeks.

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BIOCHEMICAL TESTS.

The amounts of chorionic gonadotropin in plasma and in urine, on average, are higher than those found with a singleton pregnancy, but not so high as to allow a definite diagnosis of multiple fetuses. Twins are frequently diagnosed during an evaluation for an elevated maternal serum alpha-fetoprotein level, although this alone is not diagnostic. Currently, there is no biochemical test that in any individual case will reliably differentiate between the presence of one and more than one fetus.

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MATERNAL ADAPTATION

Nausea and vomiting
The normal maternal blood volume expansion is greater in twin pregnancies. Whereas the average increase in late pregnancy is about 40 to 50 percent with a single fetus, it is about 50 to 60 percent with twins, which amounts to a maternal blood volume about 500 mL greater.
The red cell mass increases as well, but proportionately less in twin pregnancies than in singletons, resulting in a more pronounced "physiological anemia."
The average blood loss with vaginal delivery of twins is 935 mL, or nearly 500 mL more than with delivery of a single fetus.

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PREGNANCY OUTCOME

ABORTION
PRETERM BIRTH (delivery before term is the major reason for the increased risk of neonatal death and morbidity in twins).
MALFORMATIONS (the incidence of congenital malformations is appreciably increased in twin and higher-order multiple gestations compared with singletons. Major malformations occur in 2 percent and minor malformations in 4 percent of twins)

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According to Schinzel and associates (1979), anomalies in monozygotic twins generally fall into one of three categories:

1. Defects resulting from twinning itself, which some consider to be a teratogenic event. This category includes conjoined twins, amorphous twins, neural-tube defects.

2. Defects resulting from vascular interchange between monochorionic twins. Vascular connections can give rise to reverse flow with acardia in one twin, or if one twin dies, intravascular coagulation with embolization to the living twin can cause defects such as microcephaly, hydranencephaly, intestinal atresia, aplasia cutis, or limb amputation.

3. Defects that occur as the result of crowding. Examples include talipes or congenital hip dislocation.

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BIRTHWEIGHT

Multifetal gestations are more likely to be characterized by low birthweight than singletons, due mostly to restricted fetal growth and preterm delivery
Two thirds of twin and even more triplet pregnancies are complicated by fetal growth restriction.
In dizygotic pregnancies, marked size discordance usually results from unequal placentation, with one placental site receiving a better blood supply than the other, but can also reflect different genetic growth potentials.

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UNIQUE COMPLICATIONS

There are a number of unique complications that occur in multiple fetuses.

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CONJOINED TWINS

1. Anterior (thoracopagus).

2. Posterior (pyopagus).

3. Cephalic (craniopagus).

4. Caudal (ischiopagus).

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ACARDIAC TWIN

Twin reversed-arterial-perfusion (TRAP) sequence is a rare (1 in 35,000 births), but serious complication of monochorionic, monozygotic multiple gestation.
In the TRAP1 sequence, there is usually a normally formed donor twin who has features of heart failure, and a recipient twin without a normal heart (acardius) and missing various other structures

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VASCULAR COMMUNICATIONS BETWEEN FETUSES

With rare exceptions, vascular communications between twins are present only in monochorial placentas
Artery-to-artery anastomoses on the chorionic surface of the placenta or vein-to-vein communications.

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TWIN-TO-TWIN TRANSFUSION SYNDROME.

Blood is transfused from the donor twin to its recipient sibling such that the donor becomes anemic and its growth may be restricted, while the recipient becomes polycythemic and may develop circulatory overload manifest as hydrops.
The donor twin is pale and its recipient sibling is plethoric. Similarly, one portion of the placenta often appears quite pale compared with the rest of the placenta.

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