Diamniotic Dichorionic Twins
Above. Schematic of the evolution of diamniotic dichorionic twin gestation. Two separate sperm fertilize two separate ova, normally generating two separate pregnancies within the uterine cavity. The placentas of each pregnancy have one amnion and one chorion (diamniotic dichorionic placentas). This type of diamniotic dichorionic twinning occurs in 80% of all such pregnancies. However, 20% of the time diamniotic dichorionic placentation can occur in one egg twins (monozygotic) if division occurs within the first three days of fertilization. When division of a single egg occurs within the first three days after fertilization, the twins will be “identical.”
Above. Evolution of monochorionic twin gestations. A single sperm fertilizes a single ova. The placental type which evolves depends upon the timing of the cleavage of the fertilized ova.
If the fertilized egg divides within the first 3 days, diamniotic dichorionic twins occur.
If the split occurs 4 to 8 days after fertilization, diamniotic monochorionic twins may occur (This is the most common type of “identical twins”).
If the split occurs 9 to 13 days after fertilization, monoamniotic monochorionic twins may occur (The twins reside in a single sac and are vulnerable to cord entanglement).
And if the split occurs at greater than 13 days after fertilization, conjoined twins may occur.
Above. Schematic demonstrating dividing membrane arrangement in diamniotic dichorionic twin gestation. Each twin contributes one amnion and one chorion to the dividing membrane. The membrane nearest to the fetus is the amnion. Therefore, the membrane is thicker than in diamniotic monochorionic twins and the layers are composed of ACCA (amnion-chorion-chorion-amnion).
Above. Placenta of diamniotic dichorionic twins. Note twin peak sign and relatively thick dividing membrane. Fetal genders are discordant.
Diamniotic Monochorionic Twins
Above. Schematic demonstrating diamniotic monochorionic twin gestation. In this type of twinning, the placenta is composed of 2 amnions (AA) which comprise the thin dividing membrane; 1 chorion encircles the gestational sac and does not contribute to the dividing membrane.
Above. Schematic demonstrating the potential vascular arrangement in diamniotic monochorionic twins. Virtually all such twin pregnancies have vascular communications between the placentas. These arrangements may be artery to vein, artery to artery, or vein to vein. The number and arrangements of these vascular communications contribute to potential complications such as twin-twin transfusion syndrome and fetal growth restriction.
Above. Diamniotic monochorionic twins. Note thin dividing membrane with T insertion into the placenta. The placenta is fused and fetal genders are concordant. The membrane is thin but visible.
Above. Diamniotic monochorionic twin gestation with discordant fetal growth and polyhydramnios in the recipient (R) sac . Note smaller donor (D) twin and the thin monochorionic dividing membrane.
Above. Diamniotic monochorionic twin gestation with discordant fetal growth and polyhydramnios in the recipient (R) sac. Moderate fetal activity is noted in this recipient fetus.
Monoamniotic Monochorionic Twins
Above. Monoamniotic monochorionic twin pregnancy, mid-trimester. Color Doppler demonstrates multiple umbilical cord loops. There is no discernible dividing membrane and the fetus is normally active.
Above. Monoamniotic monochorionic twin pregnancy, mid-trimester. Color Doppler again demonstrates multiple umbilical cord segments. It is difficult to discern knots within the umbilical cords.
Above. Monoamniotic monochorionic twins. Color Doppler defines multiple segments of umbilical cord within the gestational sac which contains no dividing membrane. The placenta is fused and the umbilical cord insertions into the placenta are in close approximation. Fetal gender is concordant.
Above. Monoamniotic monochorionic twin pregnancies with similar findings as noted above.
Above. Color Doppler, monoamniotic monochorionic twin pregnancy, mid-trimester. The amount of amniotic fluid appears to be reduced and fetal extremities are adjacent to the multiple umbilical cord segments.
Twin to Twin Transfusion Syndrome
Above. TTTS with “stuck” donor twin.
Above. TTTS. This video demonstrates fetoscopic laser photocoagulation of placental vessels in twin to twin transfusion syndrome utilizing the Solomon Technique.
Above. First trimester, probable conjoined twins. Initial criteria include monoamniotic monochorionic membrane and monoamniotic monochorionic sac. Note on several views, twins appear inseparable. Yolk sac is visible. False positives occurred during the first trimester and follow-up scan is warranted.
Above. Conjoined Twins. Transverse views demonstrating the spines of Twin A and Twin B. Abdominal structures including the liver are shared. Transverse views crainally also demonstrate shared thoracic structures including the heart.
Above. Conjoined Twins. Transverse views demonstrating the spines of Twin A and Twin B. A single cardiac structure is shared between the twins.
Above. Conjoined Twins. Color Doppler demonstrating umbilical arteries around the bladder of Twin A, which demonstrates a 3 vessel cord. Careful observation demonstrates the bladder of each twin. The images suggest a designation of omphalopagus.
Above. Conjoined Twins. Note designations of spine for Twin A and Twin B. Shared organs include the bowel and liver.
Above. Conjoined Twins. Each fetal head is separate, but there is sharing of cardiac and abdominal structures with a likely designation of omphalothoracopagus.