The Timing of Twinning

For over sixty years it has been widely accepted that twinning, the process that results in identical or conjoined twins, normally occurs about two weeks after fertilization, or conception. This assumption has been used as a premise in what philosophers call the "twinning argument." The idea behind the twinning argument is that since one thing cannot be identical to two things, a twin cannot be identical to a zygote, or fertilized egg. Arguing from that to the general conclusion that none of us is identical to a zygote is more complicated and involves the further premise that it is not fully determined at the time of fertilization whether a zygote will undergo twinning.

If the twinning argument is sound, then it is false that you were once a zygote. This conclusion has been used as a premise in arguments defending the morality of things like contraception, the day-after pill, IVF and stem cell research.

In an article forthcoming in the journal Zygote, medical researcher Gonzalo Herranz from University of Navarre in Spain argues that there is not enough evidence to support the current model that proposes that twinning occurs two weeks after fertilization. Most of the argument here turn on the incompleteness of the theories of exactly how twinning occurs.

Herranz goes on to call attention to an alternative model of twinning, originally presented by López-Moratalla and Cerezo. According to this model, twinning occurs at the time when the zygote first undergoes cell division. The zygote can undergo two types of cell division: it can divide into a mass consisting of two blastomeres (cells) or turn into two zygotes. Twins result only in the second case.

If this model is correct, then the twinning argument is unsound. On the new model, we might well be identical to the zygote from which we developed. If the zygote from which we came is indeed identical to us, one could question the morality of things like contraceptive pills, the day-after pill, stem cell research and IVF (which produces more embryos than are implanted). There are, of course, lots of other considerations that go into determining whether preventing the development of an embryo is ethical, but the new model would complicate at least one line of defense of these practices.

But how realistic is the second model? To answer that question, we need to look closer at what happens after fertilization. Right after a sperm fertilizes an egg cell, the zygote starts undergoing cell division while traveling to the uterus, a journey that takes 5 to 7 days in humans. At first the cells form a loose mass surrounded by a membrane inherited from the egg cell, called the zona pellucida. At the 8-cell stage, the cells are tied together with proteins that allow for an exchange of chemicals.

When there are about 60 cells present (day 5), a clear visible separation takes place between the inner cell mass and an outer ring of cells called the trophectoderm. This outer ring of cells now functions as the membrane, and the thin membrane inherited from the egg cell disintegrates. The inner cell mass floats inside this membrane in a liquid that has formed during cell division. The whole thing at this stage is called a blastocyst.

During implantation, which occurs the following week, the blastocyst embeds into the tissue of the uterine wall. The inner cell mass then gives rise to the embryonic disc, which consists of two types of cells, one type becomes the embryo and the umbilical cord and the other becomes the outer membranes and the placenta.

At around day fourteen a process called gastrulation takes place. Gastrulation begins with a massive migration of cells to the center of the embryo. Part of the mass of cells folds to create a hollow and then the sides around this hollow close together to form a genuine cavity corresponding to the gastrointestinal tract. This allows a further differentiation of the cells in the inner cell mass that are specialized to become the embryo and the umbilical cord. One area of cells leads to skin, hair, nails, the lens of the eye, the nasal cavity, the sinuses, the mouth and nervous tissue. A second area of cells will go on to form the tonsils, larynx, trachea, lungs, and the lining of the alimentary tract. A third area of cells will become the muscle and connective tissue, blood cells, bone marrow, skeleton and reproductive organs.

On the traditional model of twinning, the twinning process typically occurs around the time of gastrulation, though evidence suggests that it occasionally may occur at the time of the formation of the blastocyst. The later twinning occurs the higher the chance that conjoined twins will develop.

The alternative model defended by Herranz suggests that twinning occurs at the stage of the zygote. On this model, while twinning occurs almost immediately after fertilization, physical separation of the cells likely occurs only later when we can visually observe that this happens, namely around the time of gastrulation.

Ethics and Morality Essential Reads

HIPAA Compliance After a Patient Dies

Can You Be a Hero?

While it may seem plausible that the first cell division results in two zygotes that continue to develop independently but in close proximity of each other, certain observations speak strongly against the alternative model. Blastocysts with two inner cell masses have been been observed in human IVF, suggesting that twinning can occur at the time of the formation of the blastocyst or earlier. However, blastocysts with two inner cell masses are exceedingly rare, making it unlikely that twinning typically occurs at this early stage of embryonic development. At best this piece evidence suggests that twinning could occur at the time of fertilization in rare cases, though not in the majority of cases.

Another challenge for the alternative model of twinning is to explain what makes the cells after the first division of the zygote different zygotes as opposed to a mass consisting of two (or more) cells. To be zygotes it seems that they must differ in a relevant way in their internal properties or be spatially separated in away that corresponds to the later physical separation around the time of gastrulation. But there is no evidence to suggest that the cells differ relevantly in their internal properties or alternatively are relevantly spatially separated. The lack of evidence for this counts strongly against the alternative model of twinning.

There is a further problem with the argument against the current model of twinning. The argument moves from the observation that we do not really know how and why twinning takes place to the conclusion that the current model of the timing of twinning is "not based on facts." This, however, is a bit of a jump. While it's true that we don't know how and why twinning takes place, the evidence does suggest that twinning typically occurs close to a couple of weeks after fertilization.