Re: Why is about 70% of the conjoined twins female?

Dear Louise,

Thanks for your question! The incidence of conjoined twins is quite rare, with estimates of their prevalence ranging from about 1 in 50,000 to 1 in 100,000 births. Conjoined twins are identical twins (also known as monozygotic twins, because they arise from the same fertilized egg, or zygote) that have failed to completely separate from each other during embryonic development. As you indicated in your question, it has been observed by a number of specialists that a large percentage of conjoined twins are female, usually around 70 to 80%. The reason for this skewed distribution of the male to female ratio is not known, though there are a few theories. On basic principles, one can imagine that either (1) conjoined twinning occurs more often in females or that (2) conjoined males are somehow less viable and are lost early in pregnancy. While there doesn't seem to be any data to support either hypothesis, the latter seems to be favored in a few of the articles that I read.

There are some ideas regarding what may cause monozygotic twins to develop abnormally and remain attached, mainly from studies in experimental animals. Exposure to pesticides containing the element selenium, manganese defieciences, exposure to butyric acid or acetone, infections, and radiation have been postulated to induce conjoined twins in laboratory animals, and may be significant in human embryology. Other theories that have been proposed include the presence of thyroid disease in the mother, as well as certain treatments for infertility. A few drugs have been indicated for their possible role in conjoined twinning including griseofulvin, an antifungal agent, and prochlorperazine, used often to reduce nausea and vomiting.

I hope this information is helpful! I have listed a few references below. Please feel free to contact me if you have any further questions.

-Nikki
nmdavis@fas.harvard.edu

References

Conjoined twins-an epidemiological study based on 312 cases. The International Clearinghouse for Birth Defects Monitoring Systems. Acta Geneticae Medicae et Gemellologiae 1991 40(3-4):325-35.

Castilla E., et al. The epidemiology of conjoined twins in Latin America. Acta Geneticae Medicae et Gemellologiae 1988 37(2):111-8.

Edmonds L. D. and Layde P. M. Conjoined twins in the United States, 1970-1977. Teratology 1982 25(3):301-8.

Milham S. Jr. Symmetrical conjoined twins: an analysis of the birth records of twenty-two sets. Journal of Pediatrics 1966 69(4):643-7.

Birth Defects, Orginal Article Series: Conjoined Twins. Volume III, No. 1. April 1967. Bergsma, D (ed.), The National Foundation-March of Dimes (publisher).