Embryonic And Foetal
Periods
Time period:
day 0 to birth
The
embryonic period is considered to be the period from fertilisa- tion to the end
of the eighth week. The period from fertilisation to implantation of the
blastocyst into the uterus (2 weeks) is sometimes called the period of the egg.
During the
period of the egg the early zygote rapidly proliferates to produce a ball of
cells that makes its way along the uterine tube towards the uterus. The
complexity of the blastocyst increases as it progresses towards the site of
implantation.
During the
embryonic period the major structures of the embryo are formed, and by 8 weeks
most organs and systems are established and functioning to some extent, but
many are at an immature stage of development. At the end of the eighth week the
external features of the embryo are recognisable; the eyes, ears and mouth are
visible, the fingers and toes are formed, and limbs have elbow and knee joints.
From the
ninth week to birth the foetus matures during the foetal period. The foetus
grows rapidly in size, mass and complexity, and its proportions change (for
example, head to trunk, and limbs). The foetus’ weight increases considerably
in the latter stages of the foetal period. Organs and systems continue in their
functional development, and some systems change considerably at birth (for
example, the respiratory and circulatory systems).
Birth in
humans normally occurs between 37 and 42 weeks after fertilisation.
The nine
calendar month gestation period is split into 3‐month periods called
trimesters. During the first trimester the embryonic and early foetal periods
occur. In the second trimester the uterus becomes much larger as the foetus
grows considerably, and symp- toms of morning sickness tend to subside. A
foetus in the third trimester turns and the head drops into the pelvic cavity
(engage- ment) in preparation for birth. Babies born prematurely during the
third trimester may survive, particularly with specialised intensive care
treatment.
Embryologists
use timings from the date of fertilisation, and all the timings in this book
will relate to that time. Embryologists studying the embryos of animals often
have an advantage in being able to fairly accurately note when fertilisation
occurred. Clinically, the date of fertilisation is more difficult to determine.
A woman’s
menstrual cycle will take around 28 days to com- plete, starting with the first
day of the menstrual period (bleed) and returning to the same point (Figure
5.1). Menstruation occurs for 3–6 days, followed by the proliferative phase for
10–12 days. Ovulation occurs around 14 days before the start of the next
menstrual period. If the released ovum is fertilised menstruation will not
occur. Fertilisation must occur within 1 day of ovulation.
The event of
the last menstrual period can be used to date the period of gestation
clinically, although the date on which fertilisa- tion took place will be
uncertain because of variability in the length of the cycle between the start
of menstruation and ovulation.
Clinically,
gestational timings are around 2 weeks longer than an embryologist’s timing
(Figure 5.2). If the embryonic period is complete at the end of week 8, a
clinician would record this as the end of week 10 (Figure 5.3).
Clinical relevance
If you are a
medical, nursing or health sciences student then you must be aware of the
2‐week difference between embryologists’ and clinicans’ gestation timings.
A gestation
period of 40 weeks is equal to 10 lunar months. A period of 10 lunar months is,
on average, 7 days longer than any 9 calendar months. Using the mother’s date
of the start of her last menstrual period you can quickly calculate an
estimated date of delivery by adding 9 calendar months and 7 days.
An awareness
of the period of the egg, the embryonic period and the trimesters helps
understand the periods of susceptibility of the embryo and the foetus. For
example, after the period of the egg and during the embryonic period the embryo
is particularly vulnerable to the effects of teratogens and environmental
insults. The respiratory system develops significantly during the third
trimester, so linking the timing of a prem e potential requirements of the baby
are important.
