Twin with one boy and one girl with single placenta

Question

I am 27,male and just become father of twins. One baby boy and one baby girl. According to my wife’s medical report both of the babies were sharing a single placenta. I am not an expert in medical science but through internet I have read that the twins will be either boy or girl. So is it a rare case that I got a baby boy and a baby girl who were sharing a single placenta? Just curious to know. If yes then what was the probability of this case?

Answer 1

First and foremost, congratulations!

Yes, it is extremely rare for dizygotic twins (two eggs/zygotes, fraternal) to share a single placenta. This is referred to as monochorionic (one placenta) dizigotic twins (MCDT). This is the rarest of all twins.

It was a well known occurrence in cattle, but until recently not recognized in humans. For this reason, articles you find online will often still state that twins with shared placentas are monozygotic (identical) twins. The rate is influenced by assisted reproductive technology (ART) e.g. IVF, which itself increases the rate of twin births. The exact rate is not known, but it is probably more common than estimated, as more testing is now being done on monochorionic twins.*

This happens because the two placentas happen to implant too closely to each other in the uterus, and sharing a relatively small space grow together to form one whole. The degree to which they grow together is variable.

Without IVF, twin birth (in the US) occurs in about 2% of births, so 1 in 50. Including IVF, twin births increase the total to about 3 in 50 births (numbers vary from country to country). In one study of 31 MCDTs, 87% followed assisted reproductive technology; only about 18% were conceived naturally. So, 18% of (2/100) is about 1 in 250 naturally conceived twin births. If ASR twins are included, it increases, but it's still uncommon.

*_{In one case, a pair of monochorionic twins were found to be dizigotic at 14 months of age after testing because the parents believed they were not identical.}

_{Unusual Twinning Resulting in Chimerism: A Systematic Review on Monochorionic Dizygotic Twins
Chimerism in monochorionic dizygotic twins: Case study and review}