Form and Function

One of my TV channels is Discovery Health which brings a lot of interesting shows my way. Recently I watched one on Twin to Twin Transfusion Syndrome which was interesting because I know probably less than I should about multiple pregnancies. So I'll start here with the most interesting (to me) part of identical twin embryonic development, specifically the way in which the number of days post conception at which splitting occurs affects various aspects of the twins' development. The listed "days post-fertilization" are approximate.

* If the zygote splits 1-3 days post-fertilization, the twins will have separate placentas and amnions. All identical twins except this type are at risk of developing TTTS.

* If the split occurs 4-8 days post-fertilization the twins will share a placenta but will have separate amnions.

* If the split occurs 9-13 days post-fertilization the twins will share the placenta, chorion, and amnion, meaning that they live in the same amniotic sac during gestation.

* If the split occurs on day 14 post-fertilization or beyond, conjoined twins will result.

Twin to Twin Transfusion Syndrome occurs in about 20% of twin pregnancies where both babies share the same placenta. It is caused when blood flow from the placenta is shared unequally between the babies or blood vessels directly connect the babies' umbilical cords. Sometimes both conditions exist. One twin, the "recipient" will receive an excess of blood, straining the heart and resulting in an excess of amniotic fluid (secreted as urine). The "donor" twin has a very low blood volume resulting in slowed growth and a lack of amniotic fluid.

Treatments are few and are frequently unable to provide a positive outcome for both twins or even for one of them. The most conventional treatment is the draining, generally repeated, of the excess fluid that collects around the recipient twin. An alternative treatment, and the focus of the show I was watching, consists of intrauterine laser treatment to coagulate blood vessels that connect the donor and recipient twins in an effort to restore equal blood flow to both babies.