# When do identical twins split?
**URL:** https://boards.straightdope.com/t/when-do-identical-twins-split/60469
**Category:** Factual Questions
**Created:** [April 2, 2001, 9:32am UTC](https://boards.straightdope.com/t/when-do-identical-twins-split/60469 "2001-04-02T09:32:38Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@flodnak](https://boards.straightdope.com/u/flodnak)
#### Post date: [April 2, 2001, 9:32am UTC](https://boards.straightdope.com/t/when-do-identical-twins-split/60469/1 "2001-04-02T09:32:38Z")
Identical twins, of course, are the result of a remarkable accident: a single fertilized egg splits completely, into two parts which are both capable of growing into a fully developed individual.
I’m working on a book in which a twin pregnancy figures fairly heavily. What I need to know to make sure this all works is _when_ twinning takes place, specifically whether it happens before or after the fertilized egg/eggs implant in the uterus. I’ve checked out some books and some Web sites on twins. I believe what I’m reading is that there is a range of time during which a fertilized egg can split into identical twins, and this spans the time during which the egg implants. Thus, twinning can take place either before or after implantation. The problem is that none of the sources that go into this much detail are written in plain English, and none of them address this detail directly.
Is there anyone here with the expertise to help me out?
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### Author:  [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)
#### Post date: [April 2, 2001, 2:49pm UTC](https://boards.straightdope.com/t/when-do-identical-twins-split/60469/2 "2001-04-02T14:49:13Z")
No expertise, just a search engine. 😃
Does this help?
[http://www.bellaonline.com/family/parenting/twins/articles/art966432527734.htm](http://www.bellaonline.com/family/parenting/twins/articles/art966432527734.htm)
> [@](#):
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> The miracle of conception takes place when an egg is fertilized by a sperm. Twinning occurs by the simultaneous development of two embryos. After fertilization, the ova moves into the uterus and implants themselves into the uterine wall.
[http://www.pbs.org/faithandreason/biogloss/zyg-body.html](http://www.pbs.org/faithandreason/biogloss/zyg-body.html)
> [@](#):
>
> Zygote
> A zygote is the product of the fusion of an egg and a sperm. It contains two copies of each chromosome, one from each parent. Egg and sperms cells, on the other hand, each contain only one copy of each chromosome. The zygote develops into an embryo.
[http://www.advancedfertility.com/zygotes1.htm](http://www.advancedfertility.com/zygotes1.htm)
> [@](#):
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> Fertilized human eggs
>
> One-cell embryos, also called zygotes
A one-cell fertilized egg is called a “zygote”. After the zygote doubles its size (not the same thing as doubling its genetic material which is what “identical twinning” is), becoming a two-celled object, it’s called an “embryo”.
This site is for starfish, but the technical terms are the same. The single cell egg is fertilized, becoming a “zygote”. It’s at this point that its genetic material can double itself (or halve itself, depending on how you want to look at it), forming identical twins. After the zygote starts growing, into two cells and four cells and eight cells, it’s an embryo and it can’t form identical twins anymore.
[http://www.uoguelph.ca/zoology/devobio/210labs/cleavage1.html](http://www.uoguelph.ca/zoology/devobio/210labs/cleavage1.html)
It can, however, form conjoined twins, depending on how the embryo implants itself.
[http://www.uhrad.com/pedsarc/peds034.htm](http://www.uhrad.com/pedsarc/peds034.htm)
> [@](#):
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> Embryology: Only monozygotic twins can be conjoined. Monozygotic or identical twins account for 30% of all twins.
>
> Four days after fertilization the trophoblast (chorion) differentiates. If the split occurs before this time the monozygotic twins will implant as separate blastocysts each with their own chorion and amnion. 25% of monozygotic twins are dichorionic. All dichorionic twins are diamniotic.
>
> Eight days after fertilization the amnion differentiates. If the split occurs between the 4th and 8th days, then the twins will share the same chorion but have separate amnions. Monochorionic diamniotic is the most common form at monozygotic twins, accounting for 75% of monozygotic twins.
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> If a split occurs after the 8th day and before the 13th day, then twins will share the same chorion and amnion. This is a very rare condition and accounts for 1-2% of monozygotic twins.
>
> The embryonic disk starts to differentiate on the 13th day. If the split occurs after day 13, then the twins will share body parts in addition to sharing their chorion and amnion.
All the “twins” websites are talking about the zygote splitting, not the embryo. Sounds to me like the answer to your question is, “Twinning takes place immediately after fertilization, while the zygote is still in the Fallopian tubes.”
[hijack]Did you see Fenris’ cry for Norwegian translation help?
[http://boards.straightdope.com/sdmb/showthread.php?threadid=65892](http://boards.straightdope.com/sdmb/showthread.php?threadid=65892)
Is this a famous Norwegian musical? I’m all agog to find out what happens. Who does she marry? Could she ever be happy with someone named Mork? Who is Soria Moria? Why is the chambermaid named after women’s underwear?
[/hijack]
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### Author:  [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)
#### Post date: [April 2, 2001, 2:54pm UTC](https://boards.straightdope.com/t/when-do-identical-twins-split/60469/3 "2001-04-02T14:54:24Z")
I mean, the twinned zygote can implant itself (themselves) creatively and form conjoined twins. But when an egg is fertilized, that’s the moment of truth–to twin or not to twin?
…and I believe some of the confusion may stem from the fact that a lot of anti-abortion websites use the terms “zygote” and “embryo” interchangeably, but in technical, biological terms they mean two quite different things.
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### Author:  [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)
#### Post date: [April 2, 2001, 3:10pm UTC](https://boards.straightdope.com/t/when-do-identical-twins-split/60469/4 "2001-04-02T15:10:00Z")
Well, dang, I’m still not saying what I meant to say. Bring more coffee. :rolleyes:
This–
> [@](#):
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> After the zygote starts growing, into two cells and four cells and eight cells, it’s an embryo and it can’t form identical twins anymore.
>
> It can, however, form conjoined twins, depending on how the embryo implants itself.
–unfortunately sounds like I’m saying that the multi-celled growing embryo with a single batch of genetic material can at some point in the proceedings still split into identical twins, either while traveling down the Fallopian tubes or after it reaches the uterus. No, sorry. My bad. This is not my understanding of the process. The moment of fertilization is when the balloon goes up, the vote is called for, the Final Answer is given. Twin or not?
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### Author:  [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)
#### Post date: [April 2, 2001, 8:36pm UTC](https://boards.straightdope.com/t/when-do-identical-twins-split/60469/5 "2001-04-02T20:36:52Z")
> [@](#):
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> \*Originally posted by Duck Duck Goose \*
> the multi-celled growing embryo with a single batch of genetic material can at some point in the proceedings still split into identical twins, either while traveling down the Fallopian tubes or after it reaches the uterus.
This is, in fact, exactly what happens:
[http://www.britannica.com/eb/article?eu=119860&tocid=63778](http://www.britannica.com/eb/article?eu=119860&tocid=63778)
After fertilization, the single-celled embryo (i.e., zygote) starts to divide into 2, 4, 8, etc, cells and moves down the Fallopian tubes into the uterus, by which time there are about 12 cells. During this time it is a simple solid ball of cells called a morula. This develops further into a hollow ball of cells called a blastocyst. At this point, some differentiation starts to occur, with some cells destined to become the placenta, while other cells will go on to form the embryo proper. Implantation occurs, of course, only after the blastocyst stage has been reached.
Now to more directly answer the OP:
[http://www.britannica.com/eb/article?eu=119860&tocid=63796#63796.toc](http://www.britannica.com/eb/article?eu=119860&tocid=63796#63796.toc)
> [@](#):
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> Three-fourths of [monozygotic twins] develop within a common chorionic sac and share a placenta; one-fourth have individual sacs and placentas. The latter condition results from a mishap before implantation, when the cleavage cells separate into two groups and then become individually implanting blastocysts.
In other words, twinning may occur before or after implantation. If it occurs before implantation, the embryo completely splits (remember its just an undifferentiated mass of cells at this point) and the two resulting embryos implant separately and develop individual placentas. If the split occurs after implantation (or, more properly, blastulation) then you get twins which share a placenta.
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### Author:  [@douglips](https://boards.straightdope.com/u/douglips)
#### Post date: [April 3, 2001, 1:01am UTC](https://boards.straightdope.com/t/when-do-identical-twins-split/60469/6 "2001-04-03T01:01:36Z")
> [@](#):
>
> _Originally posted by Duck Duck Goose \*
> \*\*
> A one-cell fertilized egg is called a “zygote”. After the zygote doubles its size (not the same thing as doubling its genetic material which is what “identical twinning” is), becoming a two-celled object, it’s called an “embryo”._\*
I think this is part of your misunderstanding, DDG. There is no process a cell can go through to divide into diploid daughter cells that is different from any other method. Specifically, a cell divides through mitosis into two identical but smaller cells. Each daughter again splits into identical but smaller cells. In this way, the number of cells in the embryo doubles with each division.
At anytime before cell differentiation (and as demonstrated above with your own posts and those of Terminus Est, after differentiation with the result of various shared parts, be they amnion/chorion or body parts) the embryo can split into two smaller groups of cells thus forming twins.
In short, there is nothing special about the first cell division as compared to subsequent divisions (before differentiation), nor is there any thing different between “Doubling the genetic material” (which I don’t think is a valid biological concept) and “doubling its size” which I think is what you are calling mitosis.
> [@](#):
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> \*\*After the zygote starts growing, into two cells and four cells and eight
> cells, it’s an embryo and it can’t form identical twins anymore. \*\*
Your own cite shows that ‘twinning’ can occur up to the 13th day, and thereafter for conjoined twins. The zygote is dividing like crazy this whole time, it isn’t a single cell for 13 days.
Finally, conjoined twins are those whose split occurs late in development and doesn’t complete, not those who split and come back together at implantation. This is described in your own cite:
> [@](#):
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> The embryonic disk starts to differentiate on the 13th day. If the split occurs after day 13, then the twins will share body parts in addition to sharing
> their chorion and amnion.
Sharing parts = conjoined.