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).
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.