Hello, and welcome to this lecture on the three
major stages of development, but focusing
on the embryonic period, from the third week
to the eighth week after fertilization.
And the reason we’re focusing on that particular
time is because this is the sensitive period
when major morphological abnormalities develop.
That’s major abnormalities affecting the
body systems, such as the limbs, the skull,
the brain, and so on. Because of this, it’s
also emotionally challenging. It’s a disturbing
subject to talk about, and therefore, we have
to be aware of that when we’re dealing with
it, both for ourselves and also for others.
In the lecture, we’ll explore this critical
sensitive period of development. Then we’ll
look at the causes of some major morphological
abnormalities, and the implications that this
may have for parents and for children as a consequence.
The three major periods of development
in human development are, first of all, the
pre-embryo, then the embryonic period, and
then subsequently, the fetal period. We have
a little calendar to this side to illustrate
that there’s time involved here and these are
of very different lengths. So the pre-embryo
is from fertilization to about two and a half
weeks later. The embryonic period is from
that two and a half weeks up to about eight
weeks after fertilization, and then the fetal
period is from nine weeks after fertilization
up to the time of birth. These are technical
terms. So people will often talk about embryos
when they really mean the pre-embryo or a
fetus when they really mean the embryonic
stage. But there’s value in having technically
defined terms in medical terminology so that
we can speak with precision. Just be aware
that it’s not the same as the common language
usage of these particular terms. You can see
there are also very different lengths. The
fetal period is much longer than pre-embryo
and embryo and occupies the major part of
the developmental process. Another thing to
point out at this point is the [inaudible 00:02:14]around the
timing of pregnancy or the time of gestation.
Since nobody knows exactly when they became
pregnant, despite the belief that you might
be able to tell that, it’s normal to time
a pregnancy from the flow phase of the last
menstrual period. On average, that’s about
two weeks before ovulation when pregnancy
can actually begin. Therefore, time of pregnancy,
as described by doctors and nurses, is the
time after fertilization plus two weeks.
So, when it’s commonly said that pregnancy
lasts for 40 weeks, that’s the time of pregnancy.
The time which you’re actually pregnant
is 38 weeks, and this can lead to confusion.
So, in these lectures, I will generally specify
the time after fertilization if that’s for
timing. Also, it’s conventional to describe
pregnancy into three trimesters, each of which
is about three months long. So, doctors or
nurses may speak about the first, second,
or third trimester. These are chronological
divisions and they do not correspond to significant
periods of change within the embryo itself
whereas, the term pre-embryo, embryo, and
fetus refer to quite specific periods of time
when very different things are happening.
For instance, in the pre-embryonic period,
that’s zero to two and a half weeks, the
pre-embryo is highly regulative.
So these are the stages at which twins can form to
completely separate, or even more individuals
can form during the pre-embryonic period.
Similarly, if some cells were damaged during
the pre-embryonic period, there’s a very
reasonable chance that the remaining cells
will just divide more frequently and make
up for that damage. So, perhaps unexpectedly
in that pre-embryonic period, the pre-embryo
is quite resistant to various kinds of damage.
However, that very quickly moves into the
embryonic period from two and a half weeks
to the end of the eighth week after fertilization.
And that’s a highly sensitive period.
It’s when the major body systems are laid down,
and at that stage, they are very sensitive
to disturbance, and therefore, this is when
major morphological abnormalities can develop.
The remaining fetal period should not be assumed
to be entirely safe from outside influences either.
During this period, tissues are maturing.
So, for instance, the bones are forming, the
teeth are forming. Perhaps most crucially,
the brain itself is developing.
Therefore, things which happen during the fetal period
may have an influence on these tissues as
they mature. So it would not bring about a
major morphological change but it could bring
about a significant functional change.