Re: Two placentas, one baby.

Thank you for a very interesting and challenging question. I have searched for the term "double placenta," and found that it is not a commonly accepted medical term because people don't agree what it means. Sometimes, a placenta appears to have two lobes. There is some speculation that some cases of what appears to be a double placenta is the result of the early loss of a twin.

To answer part of your question directly, in the case of monozygotic twins, there are three possibilities for the extraembryonic structures. With early twinning, there will be two chorions and two amnions; even in these cases, it appears that the two placentas fuse about half the time. There are also cases with two amnions and one chorion, and finally, cases with one chorion and one amnion. Please see Figure 11.32 of Gilbert's Developmental Biology from the Books search at NCBI. Go to:

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Books

Search for "twinning" and find the figure from Gilbert.

Here is a report of a double placenta and bifurcated umbilical cord in a singleton pregnancy:

http://jdm.sagepub.com/ cgi/content/abstract/17/5/280

Finally, there is an excellent review on chimerism here:

http:// humrep.oxfordjournals.org/cgi/content/full/21/3/579

After reading this review, it seems to me that there is a great deal of dogma about twinning that cannot be correct.

I think that it is fair to summarize the possible causes of "double placenta" this way:

1) The placenta might appear to have two lobes in a singleton birth.

2) The "double placenta" might result from the early loss of a twin, or from the incomplete separation of twins in an apparent singleton birth of a chimeric individual.

Boklage, 2006, (Human Reproduction 21: 579, link provided above) concludes:

"It is clear that most twin conceptions do not result in twin births. Survival of both members of a pair of twins from fertilization to term is rare (1 in 50 in apparently optimal circumstances). There is a sole survivor from 25% of twinning events and none from the rest. Sole survivors of twin conceptions are several times more common among live births than twins. By conservative estimate, sole survivors of multiple conceptions are at least as frequent as one live birth in eight (Boklage, 1990, 1995), roughly 10 times the frequency of twin pairs among all deliveries. Given that most spontaneous human chimeras discovered to date have been under the lifelong impression that they had always been singletons, there is no reason to suppose chimerism would be less frequent among sole survivors of DZ conceptions than it is among liveborn DZ twins (van Dijk et al., 1996). We must infer that most chimeras are born single."

Although there is not a detailed discussion of placental morphology in this review, it appears that the chimerism is common. The tool used to detect chimerism are relatively insensitive. Chimerism usually has no medical consequences, so very little effort is put into detecting it.

The normal incidence of twins is about 1 in 90 births (1.1%). Sole survivors of twinning events are several times more common. According to Boklage, 2/100 twinning events lead to twin births; 25/100 lead to sole survivors. This would make the frequency of singleton survivors of twinning events about 13.8% (this is about the 1/8 of Boklage). You could very well be a chimera; you almost certainly know several of them.

Thank you again for an interesting question!

Yours,

Paul Szauter
Mouse Genome Informatics