Around 80% of monozygotic twins originate from monochorionic/diamniotic pregnancies, where they share the same placenta in the womb. MZ twins are recognized to be at greater risk of congenital anomalies. One of the most common complications is severe discordant growth, in which there is a 25% or greater difference in fetal or birth weight between the twins in the absence of twin-to-twin transfusion syndrome.
Such severe discordant growth has been reported in 7% to 14% of all monochorionic/diamniotic pregnancies. Unequal placental sharing can explain about half of these pregnancies; the cause of the other half is largely unknown.
The case presented in the latest paper, published in the journal Stem Cell Reports, suggests that uneven growth of identical twins could start during the early stages of preimplantation development. An embryo donated for research revealed two inner cell masses (ICMs, internal cluster of cells at the embryonic pole of the blastocyst which develops into the body of the embryo), which is a sign of monochorionic/diamniotic pregnancy that will give rise to MZ twins.
In this case, the two ICMs were not equal and high-resolution RNA sequencing indicated that they were at different stages of development; the smaller one was in the earliest stages of cell lineage commitment, while the bigger one had already differentiated to primitive endoderm. IGF1-mediated signaling is likely to play a key role in ICM growth and to be the major driver behind these differences.
The differences in development would become negligible; however, the difference in size would be likely to continue throughout the pregnancy.