Twins may be monozygous (from the same ovum) or dizygous (from two separate ova). Examination of the placenta can help to determine the likely twin zygosity.
Twin placentas are classified as:
(1) separate placental plates (two singleton placentas)
(2) fused placentas, which has a chorionic layer in the membrane separating the two gestational sacs (diamniotic, dichorionic)
(3) single placental plate with two amniotic sacs but no chorionic layer in the dividing membrane between the sacs (diamniotic, monochorionic)
(4) single placental plate with no dividing membrane between the two fetuses (monoamniotic, monochorionic)
Most twin pregnancies are dizygotic:
(1) approximately 70% are dizygous (DZ).
(2) The remaining 30% are monozygous (MZ).
Frequency Data for placental type vs zygosity (after Benirschke and Kim, 1973)
Percent of all twin placentas
Percent of Monozygous Placentas
Percent of Dizygous Placentas
20% of MZ (6 of 30)
60% of DZ (42 of 70)
13.3% of MZ (4 of 30)
40% of DZ (28 of 70)
63.3% of MZ (19 of 30)
0% of DZ (0 of 70)
3.3% of MZ (1 of 30)
In order to calculate the probability for MZ or DZ in separate and diamniotic-dichorionic placentas, the frequency of same sex DZ pregnancies is needed (all of the MZ pregnancies should be same sex). If one assumes that DZ pregnancies 25% male-male, 25% female female and 50% male-female, then 87.5% of separate placentas with same sex babies are DZ (21 of (6 + 21)) and 78% of diamniotic dichorionic placentas are DZ (14 of 18 (4 + 14)).
Some rules for assigning zygosity:
(1) Twins of unlike gender are considered dizygotic; a rare exception is the presence of heterokaryotypic monozygotic twins.
(2) Monochorionic placentas indicate monozygosity.
If there is a single placental plate and two amniotic sacs are present:
(1) if the sacs consist only of amnion (monochorionic), then they can be readily stripped from the placental surface, leaving no residual line of demarcation
(2) if a chorionic layer is present in the common wall (dichorionic), it cannot be easily stripped since it is continuous with the chorionic plate
When in doubt, restriction fragment length pleomorphism (RFLP) analysis of DNA can be performed.
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Specialty: Obstetrics & Gynecology