Mechanisms of twinning. III. Placentation, calcium reduction and modified compaction.

OBJECTIVE

To elucidate phenomena contributing to the maintenance and persistence of early uniovular pregnancies. It is proposed that the decreased survival of monozygotic (MZ) multifetal gestations is related to a smaller uteroplacental junctional (UPJ) area during and following implantation when compared with dizygotic (DZ) pregnancies.

STUDY DESIGN

Pertinent placental data were collected and analyzed. From this information the placental area was calculated, and an estimate of the size of the UPJ was derived.

RESULTS

In the first part of the study, MZ twins, triplets, quadruplets and quintuplets displayed a decrease in mean placental area per neonate as the number of fetuses increased in each set. In the second part, as predicted, MZ twin placentas exhibited smaller placental junctional areas than DZ. This is seen as a direct result of decreased trophectoderm mass due to scission in early gestation.

CONCLUSION

These data are consistent with the general impression that multifetal pregnancies, especially monozygotic, are more at risk than singletons because of diminished placental support per fetus. They also reinforce the proposal that the key to MZ twinning is depressed calcium levels.