Human Anatomy
Introduction to Embryology
Introduction to embryology for MBBS: stages of human life, embryonic and fetal periods, trimesters and key terms such as ontogeny, phylogeny and viability, mapped to NMC codes AN76.1 and AN76.2.
MedNext Academy | 3 min read
Introduction to Embryology
Introduction to embryology for MBBS: stages of human life, embryonic and fetal periods, trimesters and key terms such as ontogeny, phylogeny and viability, mapped to NMC codes AN76.1 and AN76.2.
This chapter introduces the language and timeline of human development, dividing prenatal life into the embryonic and fetal periods and the three trimesters. It defines key terms such as ontogeny, phylogeny, trimester and viability.
High-yield: Introduction to Embryology
- Embryology is the study of the development of the individual from fertilisation to birth.
- Prenatal life is divided into the embryonic period of the first eight weeks and the fetal period thereafter.
- The embryonic period is when the organ systems form and the embryo is most vulnerable to teratogens.
- The fetal period is a time of growth and maturation of the organs already laid down.
- The whole gestation is divided into three trimesters of about three months each.
- Ontogeny is the development of the individual, while phylogeny is the evolutionary development of the species.
- Viability is the stage at which a fetus can survive outside the uterus, at about 24 weeks with modern care.
- The germinal stage covers fertilisation and the first two weeks up to implantation.
- The stages of human life run from the zygote and embryo through the fetus, infant, child, adolescent and adult.
- The first two weeks follow the all-or-none rule, where a severe insult either destroys the embryo or is fully repaired.
- The third to eighth week is the period of organogenesis and the most critical time for structural birth defects.
- A teratogen is any agent that can disturb development and cause a congenital malformation.
- The last menstrual period is commonly used to date a pregnancy, giving a term of about 40 weeks.
- Fertilisation age is counted from conception and is about two weeks less than the menstrual age.
- Understanding normal development is the foundation for recognising congenital anomalies in clinical practice.
Timeline of prenatal development
- **Germinal stage:** Weeks one to two: fertilisation to implantation; all-or-none response to injury.
- **Embryonic period:** Weeks three to eight: organogenesis; greatest vulnerability to teratogens.
- **Fetal period:** Week nine to birth: growth and maturation of formed organs.
- **Key terms:** Ontogeny is individual development, phylogeny is species evolution, viability is survival outside the uterus.
NMC competencies in this chapter
- **AN76.1:** Stages of human life
- **AN76.2:** The terms phylogeny, ontogeny, trimester and viability
Frequently Asked Questions
What is the difference between the embryonic and fetal periods?
The embryonic period is the first eight weeks, when the organ systems form and the embryo is most sensitive to teratogens. The fetal period follows and is mainly a time of growth and maturation.
What do ontogeny and phylogeny mean?
Ontogeny is the development of an individual from fertilisation to maturity, whereas phylogeny is the evolutionary development of a species over time.
What is viability?
It is the stage of development at which a fetus is able to survive outside the uterus, generally taken to be around 24 weeks of gestation with modern neonatal care.
Why is the third to eighth week so important?
It is the period of organogenesis, when most organs are being formed. A teratogenic insult during this window is most likely to cause a major structural birth defect.
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