It is indeed rare for fraternal twins of different genders to share a single placenta, as this typically indicates identical twins of the same gender. Fraternal twins usually have two separate placentas, and the occurrence of a shared placenta in boy and girl twins is an unusual event in embryology.
Twins Sharing a Single Placenta
The scenario you’ve described, where twins share a single placenta and yet are of different genders, is indeed a rare case. Typically, twins that share a placenta, also known as monochorionic twins, are monozygotic (identical) and thus they have the same genetic makeup, which means they should be of the same sex. In human reproduction, the gender of a baby is determined by the sex chromosomes; females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).
Having a boy and a girl (fraternal twins) sharing the same placenta can happen if there is a fertilization of two different eggs (dizygotic twins) that implants very close to each other in the uterus. In such a case, the placentas can fuse together and appear to be a single placenta. This occurrence is exceptionally rare. While the probability is not precisely determined, the occurrence aligns with other highly uncommon reproductive events and would be significantly less frequent than the common cases of fraternal or identical twins.
Given that monozygotic twins are always of the same sex because they originate from a single zygote that splits, having bichorionic dizygotic twins where each has their own placenta is much more common when the babies are of different sexes. If indeed your twins are a boy and a girl sharing one placenta, this would be considered a rare event in twin pregnancies. Such cases may pique the interest of medical researchers, as these instances can offer insights into embryonic development and placental formation.