become pregnant. Now, of course, we can also
have twinning. About one pregnancy in 90 will
give rise to twins. And about a third of the
twins are identical twins. But there are quite
a lot of variations in the numbers of twins,
and that’s as a result of in vitro fertilization
or assisted reproductive techniques. It
has been common to re-implant more than one
fertilized egg, typically, two or sometimes
three. And if both are all three of these
take, if they all implant and give rise to
a pregnancy, then you’ll significantly increase
the number of non-identical twins as a result.
Twin pregnancies are always that little bit
more challenging than a normal pregnancy.
So the chance of something going wrong is
increased in a twin or multiple pregnancies
and therefore, that in itself can have negative
consequences. And that’s why largely, as a result
of that observation, in vitro fertilization
clinics are increasingly moving towards
the idea of just returning one fertilized
egg as a result. Different kinds of twins
can result in different arrangements of the
extra-embryonic tissues. So, you can see in
the left-hand side of the diagram that there
has been a division very early on, under
two completely separate blastocysts, which
will go and implant completely independently.
So, these twins will have a completely independent
set of extra-embryonic membranes within the
uterus. If, however, fertilization gives rise
to twinning at a particular stage or the inner
cell mass cell stage, then each of those inner
cell masses can give rise to an embryo, and
each of them will then share membranes with
each other. And there are technical names
for each of those which we’ll explore in
another lecture. If, however, two embryos
were really close together, this probably
happens round about the primitive streak stage.
If they are developing close together and
come in contact with each other, then you
may well get conjoined twins. Twins can be
conjoined in a variety of different places,
at the head, for instance, or in the midline
in the abdomen as a result of coming too close
together at some point during the early stages
of development. So then, in summary, what
have we looked at in this lecture?
We’ve been looking at the second week of the pre-embryo.
We saw the formation of the primitive streak
and gastrulation, which gives rise to the
main body axis, and also, the three body
layers, ectoderm, mesoderm, and endoderm.
Subsequently, the neural tube begins to form
in the process of neurulation, and if that
is incomplete, if that remains open at any
point, then you’ll have a neural tube defect.
That could be at the head end as an anencephaly,
or it could be further down the spine as
in spina bifida. Spina bifida defects may
be open or closed, and the severity depends
on your nature. The more open, the more serious
the defect is. Finally, we looked briefly
at the possibility of twinning and the different
arrangements of extra- embryonic membranes that
you can get as a result of the twinning process.
Thank you very much.