What triggers twinning?
Q: In the case of identical
twins, what triggers a single zygote to result in two embryos? What
is the probability of having identical twins?
A: Your first question has me stumped. I've
asked medical experts from various universities and hospitals. The
general consensus is: we don't know.
Identical twins do not run in families and, therefore, twinning
is not related to genetics. Comparisons among different ethnic
groups rule out an ethnic cause-no difference. Only mother's age
correlates with twinning: the older the mother, the greater the
I can mention how and when two embryos form
instead of one even though we don't exactly know what triggers the
events. We have some hunches though, which I'll mention as we
discuss the timing. Twinning is abnormal in humans. Things can go
wrong at different times during the embryo's development.
During the first four days after fertilization, the huge
fertilized egg (zygote) splits into many smaller cells suitable for
building the eventual baby. Sometime after the fourth day about 30
of these smaller cells are hanging around. The spaces between them
have filled with pools of clear fluid and gather together to form a
hollow cellular ball, called a blastocyst. An inner cell mass forms
inside. This is the stuff that an embryo will develop from.
Now consider the approximate time sequence (from fertilization)
of when things can go wrong and what kind of twins result.
- Day 1: the zygote has only split into two smaller
building-block cells. If the 2-cell mass splits into two clumps
now, then twins result that are the most identical. These twins
always have separate placentas since the split into twins occurs
at such an early stage. Doctors often mistakenly identify such
twins (with separate placentas) as fraternal twins even though
they are the most identical twins that can happen. Ironical.
- Day 4 to 5: Most twins form at this time. What happens at this
time? The blastocyst and its inner cell mass (i.e., the developing
embryo) must "hatch out of a shell" says Harvey Kliman, M.D.,
Ph.D. at the Yale University School of Medicine, much like a chick
hatches out of its shell.
can see this happening in the photograph of Figure 1. Michael
Tucker, PhD., F.I. Biology, captured a hatching event with the
incredible photograph. The shell (a gelatinous capsule called the
zona pallucida) is the fuzzy layer around the blastocyst. The inner
cell mass (the proto-embryo) is just starting to hatch through the
hole in the shell.
Tucker, IVF.com] Hatching blastocyst
When the whole blastocyst passes through its shell, the inner
cell mass can fragment into two (or very rarely three) clumps of
cells. The two cell clumps form into complete embryos, which become
identical fetuses, and eventually twin babies. The twins are mirror
images of each other. Dr. Kliman's daughters are mirror-image twins,
one having a set of moles on her right side and the other with the
same set of moles on her left.
The reason age contributes to the likelihood of a woman having
twins is that the enclosing shell (that the blastocyst hatches out
of) is harder in older mothers. It's more likely,
therefore, that the inner-mass will break into clumps as it hatches
out of a tougher shell.
- Day 9 to 10: After this time, if the inner cell mass divides
into two clumps, the twins share the same inner sac. This luckily
happens only 1 % of the time because the umbilical cords may twist
together then, which can lead to death.
- Day 13 to 15: If the inner cell mass splits into two masses
after Day 13, then the cells forming the two embryos don't totally
separate and the twins are born joined together, called conjoined
or Siamese twins.
The second part of your question is relatively easy. About three
in every 1000 deliveries results in identical twins. This average
seems to be the same worldwide and the reasons are uncertain.
(Answered by April Holladay, science correspondent, May 9,
Yale U: Behind every healthy baby is a healthy placenta
U of Pennsylvania: Basics of embryo development
Children's Hospital of Iowa: Twins, a parents' guide
IVF.com: Freezing human eggs