Meet the Robinsons. Violet and Rosalie are lively twin girls. I met their parents, Emily and Brett, when they were referred to me at about 20 weeks of pregnancy due to a serious complication – Twin-Twin Transfusion syndrome (TTTS). To understand this condition, it’s helpful to know something about twin pregnancies in general.
Twinning can happen in several ways.
In most cases of twins, two separate sperm fertilize two separate eggs, so there are two embryos from the start. These twins are non-identical. They are called dizygotic twins (two embryos). They are also dichorionic (they form two placentas) and diamniotic (two amniotic sacs).
In other cases, a single sperm fertilizes a single egg, and then the embryo divides in two. These twins are monozygotic, or identical; they started as one embryo. If this embryo divides in the first few days after fertilization, the embryos will land in separate places in the uterus and form two placentas and two sacs (they are dichorionic/diamniotic, or di/di, like dizygotic twins only genetically identical).
If this division happens a day or two later, during implantation, they will share a placenta but have two sacs (mono/di). If the embryo divides a day or two later, the twins will share both placenta and sac (mono/mono). And if the division happens after that, the twins will be conjoined.
Confusing, I know.
Meet the Robinson’s
Now back to the Robinson twins. They were monochorionic/diamniotic (mono/di); they shared a placenta but had separate amniotic sacs.
On their first ultrasound in our office, they had findings which were very concerning to me. One fetus, Violet, was considerably smaller than her sister. She had almost no fluid in her bladder or in her amniotic sac, and very little circulation through her umbilical cord.
Her sister Rosalie was bigger, had a large amount of amniotic fluid, and had a thicker umbilical cord. I explained to Emily and Brett that these findings were suggestive of TTTS. If the condition was not addressed promptly, the mortality risk for both fetuses was very high.
Remember that the placenta contains all the blood vessels that connect mom’s circulation to that of the baby (or babies).
In monochorionic twin pregnancies (those sharing a placenta), connections frequently develop between the blood vessels in the shared placenta, so that in addition to mom sharing blood flow with each of the babies, the babies begin to share blood flow with each other. In the case of TTTS, those connections become unbalanced.
A dangerous situation
In other words, the flow of blood in the placenta shifts so that one fetus (the recipient twin) receives too much blood, and the other (the donor twin) doesn’t receive enough. This is a dangerous situation for both fetuses.
Obviously, the donor twin is in trouble – it is not receiving enough nutrients and oxygen to grow and develop normally. The recipient twin is in trouble, too. This fetus is overwhelmed with blood flow, and its heart must work harder to move it around.
The Robinson girls were in serious condition. What were their treatment options? Treatment options can be a complex topic. Emily and Brett were stunned. They were a young couple, newly married and far from their families. The pregnancy that began as such a blessing had now taken a frightening turn. I explained that there are several options for treating TTTS.