Fraternal (dizygotic) twins will have two placentas. Sometimes, the placentas will fuse and will appear as one placenta on ultrasound and on visual inspection. Each dizygotic twin will come from a different fertilized sperm and egg. Identical twins (monozygotic), however, come from one egg and one sperm. Identical twins will have one placenta. Oftentimes, an ultrasound can tell if the twins are fraternal or identical by identifying one or two placentas. As mentioned previously, however, fraternal twins can sometimes appear to have one placenta.
The chorion is the outer membrane and the amnion is the inner membrane. All fraternal twins have their own amnion and their own chorion. This anatomical condition is known as diamniotic, dichorionic or “di/di”. All fraternal twins are dichorionic. Identical twins have a 20-30% incidence of a di/di configuration. 60-70% of identical twins will have separate amniotic sacs (monochorionic/diamniotic). 1-2% will have the same amniotic sac (monochorionic/monoamniotic). Monoamniotic twins are identical and always have one chorion. They do, however, have separate umbilical cords from the same placenta. Monochorionic-diamniotic (mono/di) twins are identical and share a placenta, but not an amniotic sac.
You can always do a DNA test if you are uncertain whether the twins are identical or fraternal. Sometimes, the doctors are confused whether the twins are identical or not as well, and may give the parents the wrong information.
Monochorionic placentas found in identical twins can be susceptible to a condition known as twin to twin transfusion (TTTS). The incidence of TTTS is about 10-15% in monochorionic placentas. This is a result of the shared vessels in the placenta which connect the placental vessels and umbilical cords. This condition cannot be prevented. With TTTS, one twin will be large while the other twin will be small. The large twin (recipient) will receive more blood volume than the small twin (donor) due to shunting of the blood in the affected placental vessels. The extra blood may place a strain on the recipient’s heart which could lead to heart failure and polyhydramnios (large volume of amniotic fluid) while the donor may develop oligohydramnios (small amount of amniotic fluid) due to poor urinary output as the amniotic fluid comes from the fetal urine. TTTS can occur at any time during the pregnancy and there are even cases when TTTS has occurred during labor. The underlying condition causing TTTS is the connection of a placental artery from one twin to the placental vein of the other.
Chronic TTTS occurs between 12 and 26 weeks gestation. Acute TTTS occurs during labor at term or in the third trimester. Twins with acute TTTS have a better chance of survival, but also have a higher possibility of surviving with handicaps.
There is also a condition known as unequal placental sharing where the clinical presentation is very similar to TTTS. The twins can have the umbilical cords implant anywhere on the placenta. As a consequence, one twin may get a greater share of the placenta than the other. This can lead to different levels of amniotic fluid and also to differences in the sizes of the twins. Unequal sharing may develop TTTS. Serial ultrasounds are recommended to follow these conditions.
A treatment for unequal placental sharing does not exist. Laser treatment or amnioreduction could potentially worsen the condition. Management consists of monitoring the twins closely, and delivering the twins early if necessary.
Amnioreduction, on the other hand, is beneficial in TTTS. Removing amniotic fluid from the recipient twin may improve circulation in the donor twin. Fetoscopic laser photocoagulation of chorionic plate vessels is very specialized and is used in cases where amnioreduction did not help. Only a few centers offer photocoagulation.
Whereas having an identical twin pregnancy is exciting, there are potential medical problems with this type of pregnancy. It is important to be followed closely by your provider due to potential complications. Twin pregnancies are also at greater risk for preterm labor, preterm rupture of membranes, gestational diabetes mellitus, and preeclampsia among others. Again, it is important to be followed closely.
Wishing you a successful and problem free delivery, Pablo.