Traditional and new models of twinning

Insights into unusual and atypical twins

Written by Assoc Prof Mark Umstad, AMBA Patron, and published in the AMBA Magazine
October 2017

For a century, it has been thought that dizygotic twins result from the fertilisation of two separate eggs by two separate sperm, and monozygotic twins form when a single egg is fertilised by a single sperm then subsequently divides to form two embryos.

In this traditional model, the timing of splitting of a monozygotic embryo determines the number of placentas and amniotic sacs. If the split occurs 1 to 3 days after fertilisation this results in dichorionic diamniotic twins, from days 3 to 8 results in monochorionic diamniotic twins and, if the split is between days 8 and 13, then monochorionic monoamniotic twins develop. If there is no split by day 13, conjoined (Siamese) twins result.

Recent studies and case reports suggest that this concept is not as simple as it seems. The traditional rules are that monochorionicity confirms monozygosity, opposite sex twins confirm dizygosity and same-sex dichorionic twins are of uncertain zygosity until postnatal zygosity testing is performed. 

Recently, this idea of splitting of the egg has been challenged on the basis that it has never been proven and never seen to occur, even with all of the technology available with in vitro fertilisation. The alternative theory is more challenging but equally unproven. The new theory contends that after the zygote initially splits, there is subsequent fusion of the placentas or membranes. The timing and degrees of fusion then determine the number of placentas and amniotic sacs. It is remarkable that in 2017 there is no proof of either theory, although the traditional model is more widely accepted. 

In the American Journal of Obstetrics and Gynecology in 2016, my colleagues and I published an extensive article that reviewed all of the evidence and literature regarding the mechanisms of twinning and reviewed unusual types of twins. This is a detailed and comprehensive scientific article but, for those who have an interest, I hope it provides the information that you require. This article can be accessed here.

"Recently, this idea of splitting of the egg has been challenged on the basis that it has never been proven and never seen to occur, even with all of the technology available with in vitro fertilisation."

In my role as Patron of AMBA I am often asked to clarify the zygosity of some twins where there may be some confusion. I am also asked to comment on atypical or unusual twins, and in the rest of this article I will provide an introduction to these ideas.

Unusual and atypical twins

Chimaeric twins
A chimaera is a single organism containing two populations of genetically different cells from two different zygotes. A zygote is an egg cell that has been fertilised by a sperm. Chimaerism has now been extensively described in humans and occurs in approximately 1:70 monochorionic twin pregnancies. In these twins, two separate zygotes fuse to form a monochorionic twin pregnancy. This means that the twins share a single placenta and can have all of the complications of monochorionic twins, including twin-to-twin transfusion syndrome. Yet, they are dizygotic so are “fraternal”. They look as different as fraternal twins and can be of opposite sex. The relevance is that all monochorionic twin pregnancies have previously been labelled as “identical” twins but this is clearly no longer always the case. 

If you feel that your twins are very different in appearance, and more likely dizygotic than monozygotic, yet the pregnancy was a monochorionic twin pregnancy, it is worth considering advanced zygosity testing. This involves testing both cheek swabs and blood from each of the twins to determine if they are chimaeric.

Mirror-image twins
Mirror-image twinning is described in monozygotic twins where their physical features are asymmetrical. That is, if one twin looks in the mirror it is like looking at his / her co-twin face-to-face. The physical features that can be asymmetrically mirrored include teeth, eye or ear features, cleft lip or cleft palate and other skin markings.

It is likely that mirror imaging results from late splitting of the zygote at approximately days 9 to 12. Left- or right-handedness is not related to mirror imaging.

Polar body twins
A polar body is a small genetic remnant following splitting of the egg before fertilisation. It has been hypothesised that this remnant of the egg splitting process could be fertilised by a separate sperm resulting in a condition called “polar body twinning” or “semi-identical twinning”. There is no evidence to support this idea.

Fetus papyraceus
A fetus papyraceus refers to a fetus in a multiple birth pregnancy that has perished early in pregnancy but can still be seen as a small disc encompassed within the placenta at the time of delivery. It usually occurs when a twin has been lost between 12 and 20 weeks and occurs in about 1:200 twin pregnancies.

There is usually no clinical significance but it can be occasionally associated with a skin condition called aplasia cutis congenita. In this condition there is a small patch of absent skin, almost always on the scalp, noted at the time of delivery. Fetus papyraceus has been reported in both monochorionic and dichorionic twin pregnancies, although it is far more common in monochorionic twins.

A fetus-in-fetu, also known as “parasitic twin”, refers to the presence of one or more partially formed fetuses situated entirely within the body of another fetus. This is an extraordinarily rare event occurring in approximately 1:500,000 pregnancies but occasionally gets significant media exposure because of the highly unusual nature of the condition.

The condition occurs during early phases when a monozygotic twin pregnancy absorbs one of the twins during the complicated process of early development of the embryo. The clinical significance is that a lump or mass may be noted anywhere within the body of a child, usually somewhere within the chest or the abdomen. Surgery is often required to remove the mass, which may be diagnostically challenging if the possibility has not been considered.

Superfetation refers to the fertilisation and implantation of a second conception during pregnancy. It is highly unlikely that this occurs. As soon as a woman becomes pregnant, ovulation is almost immediately suppressed and it is highly unlikely that a further conception can occur once a pregnancy is underway. 

Superfetation is often suspected when there is a significant difference in size between twins at the time of birth, but of course this can occur for many reasons, of which superfetation is highly unlikely.

Superfecundation refers to the fertilisation of two eggs via separate instances of sexual intercourse in the one menstrual cycle and is well described. Monopaternal superfecundation occurs after sex with one partner on separate occasions. Heteropaternal superfecundation occurs after sex with multiple partners on separate occasions. Superfecundation can also occur during IVF with simultaneous spontaneous and assisted conceptions in the same cycle.

Twinning is a far more complex phenomenon than we previously understood. The conventional model of egg splitting is the more popular and widely held, although it is possible that some element of fusion of embryos underlies monozygotic twinning. The unusual and atypical twins described in this article are very rare, but hopefully this provides some explanation for situations that may be quite difficult to understand.