Mechanisms of twinning. V. Conjoined twins, stem cells and the calcium model.

OBJECTIVE

To examine the possible role of calcium in conjoined twinning.

STUDY

DESIGN

Conjoined (CJ) twins are an extreme example of monozygotic (MZ) twinning. Data were collected from various sources on factors to which gravidas who deliver CJ twins were exposed around the time of conception.

RESULTS

A significant number of such women were subjected to environmental triggers. Of particular interest is increased CJ twinning following use of oral contraceptives. It is hypothesized that this resulted from prolonged ovulatory dysfunction in lightweight women. The incidence of uniovular twinning is inversely related to women's prepregnancy weights.

CONCLUSION

These observations are consistent with the proposed general model of MZ twinning. Factors that induce calcium depression and delayed implantation encourage uniovular duplication in general and CJ twinning in particular.