PSYB20H3 Chap.7 Prenatal Development and Teratogens
Prenatal Development and Teratogens
Three stages with fixed boundaries
The second half of Lecture 3 in PSYB20H3 divides prenatal development into three stages, and most questions ask you to place an event in the correct one. The germinal stage, also called the period of the zygote, runs from conception to implantation, about the first two weeks. The embryonic stage runs from week three to week eight. The fetal stage runs from week nine to birth.
Learning the boundaries before the milestones makes the milestones much easier to sort.
The germinal and embryonic stages
After the egg is released and fertilized, the zygote travels down the fallopian tube, divides to form a blastocyst and implants in the uterine wall.
The inner layer of the blastocyst becomes the embryo and the outer layer forms the support structures: amnion, chorion, placenta and umbilical cord. Three of every four zygotes fail to survive this phase.
In the embryonic stage the major organs form from three layers: ectoderm for the nervous system, skin and hair; mesoderm for muscles, bones and circulation; endoderm for the digestive system, lungs, urinary tract and organs such as the liver and pancreas.
Month-by-month milestones
In the first month the neural tube forms from a fold in the ectoderm and the heart begins beating.
The second month brings ears, a rudimentary skeleton, developing limbs and rapid brain growth. In the third month, the first of the fetal stage, ultrasound can detect sex and the digestive and excretory systems begin working together. Months four to six bring thumb-sucking, kicks the mother may feel, a heartbeat audible by stethoscope, vernix and lanugo.
Months seven to nine are the finishing phase, with maturing organs, weight gain, a typical age of viability of 24 weeks within a 22 to 28 week range, and a head-down position in the ninth month.
Teratogens and timing
Teratogens are environmental agents, including viruses, drugs, chemicals and radiation, that can harm a developing embryo or fetus.
Each developing structure has a sensitive period when it is especially susceptible, which is why the rapid organ formation of the embryonic period carries the greatest risk. Rubella can cross the placenta and is most dangerous in the first trimester. Thalidomide, marketed for morning sickness after showing no effect in rats, caused severe malformations when taken early in pregnancy, yet many exposed babies were unaffected.
Radiation, chemicals and pollutants, including a father's exposure, can also affect children.
What earns marks here
Ordering questions, stage-placement questions and swapped-definition items dominate. A strong written answer names the stage for a given week, states what is developing then, and links the risk of a teratogen to the sensitive-period principle before listing effects.
What this chapter covers
- 01
Germinal stage and implantation
- 02
Support structures
- 03
Embryonic stage and three cell layers
- 04
Fetal stage milestones
- 05
Vernix and lanugo
- 06
Age of viability
- 07
Teratogens and sensitive periods
- 08
Rubella and thalidomide
Worked example · free
Place three moments on the prenatal timeline
- 1(a) Week 2 is the germinal stage, which ends when the blastocyst implants in the uterine wall.
- 1(b) Week 7 is the embryonic stage, when major organs form from the ectoderm, mesoderm and endoderm.
- 1(c) Week 26 is the fetal stage and lies inside the 22 to 28 week band of viability.
Key terms
- Blastocyst
- The blastocyst is the ball of cells that implants in the uterine wall, its inner layer becoming the embryo.
- Ectoderm
- The ectoderm is the outer layer of the embryonic disk, which becomes the nervous system, skin and hair.
- Age of Viability
- The age of viability is the point, between 22 and 28 weeks and typically 24, at which survival outside the uterus becomes possible.
- Vernix
- Vernix is the white, cheesy covering that protects fetal skin from chapping.
Prenatal Development and Teratogens FAQ
What are the three stages of prenatal development?
The germinal stage runs from conception to implantation, roughly weeks zero to two. The embryonic stage runs from week three to week eight and is when the major organs form. The fetal stage runs from week nine to birth, when organ systems begin to function and the fetus grows rapidly.
Why are teratogens most dangerous early in pregnancy?
Each developing structure has a sensitive period when it is especially susceptible to damage, and most structures form during the embryonic stage. Once a body part is fully formed it is less susceptible. Rubella, for example, is most dangerous in the first trimester, when organs are forming.
What is the difference between vernix and lanugo?
Vernix is a white, cheesy covering that prevents the fetus's skin from chapping. Lanugo is a fine layer of body hair that helps the vernix stay on the body. Both appear during months four to six, and review questions often swap their definitions to test attention.
Exam move
Draw a week-by-week line from conception to birth and mark the stage boundaries, then add every milestone from memory and check them against your notes. Rehearse one teratogen answer aloud using the order stage, development, sensitive period, then effects.
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