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Here is a complete exam-focused summary of Embryology Lecture 11 - Fetus and Placenta (Geomedi, 2026):
Embryology L5 - Fetus & Placenta: Final Exam Summary
PART 1: PLACENTA
1. Placental Components (Fetomaternal Organ)
The placenta has two components:
| Component | Origin |
|---|---|
| Fetal component (chorion) | Trophoblast + extraembryonic mesoderm (chorionic plate) |
| Maternal component | Uterine endometrium (decidua) |
2. Changes in the Trophoblast
- By the beginning of the 2nd month: trophoblast shows many secondary and tertiary villi giving a radial appearance
- Capillary system in villous stems contacts capillaries of the chorionic plate and connecting stalk, forming the extraembryonic vascular system
3. Placental Development
- Maternal spiral arteries erode and release blood into intervillous spaces (via endovascular invasion by cytotrophoblast cells)
- Deoxygenated maternal blood drains via endometrial (uterine) veins between the spiral arteries
- From the 4th month: cytotrophoblast begins to disappear
- By 5 months: cytotrophoblast is essentially gone
4. Placental Tissues - Decidua & Chorion
Decidual cells form 3 regions:
- Decidua capsularis - overlies the embryo and chorionic vesicle
- Decidua basalis - between chorionic vesicle and uterine myometrium (implantation site) - becomes the placenta
- Decidua parietalis - remaining decidualized endometrium not involved in implantation
Chorionic villi develop asymmetrically:
- Chorion frondosum - flourishing villi, becomes the placenta
- Chorion laeve - smooth, remainder of the chorion (no villi)
By midpregnancy:
- Decidua capsularis fuses with decidua parietalis
- Chorion laeve and decidua capsularis undergo progressive atrophy
5. Mature Placenta (Key Numbers to Memorize)
| Feature | Value |
|---|---|
| Thickness | ~3 cm |
| Diameter | ~20 cm |
| Weight | ~500 g |
- Fetal blood is contained within blood vessels
- Maternal blood is a free-flowing lake not bounded by vessels (bathes chorionic villi)
Exchange across chorionic villi:
- Mother → Fetus: O₂, nutrients, hormones, metabolites
- Fetus → Mother: CO₂, urea, wastes, hormones
6. Hormones
- The syncytiotrophoblast is the key endocrine organ of the placenta
- It produces protein and steroid hormones
- First protein hormone: human chorionic gonadotropin (hCG) - produced even before implantation
- hCG in maternal urine = basis for pregnancy tests
PART 2: FETUS
7. Fetal Period & Measurements
- Fetal period: from the beginning of week 9 to birth
- Crown-Rump Length (CRL) = sitting height
- Crown-Heel Length (CHL) = standing height
Length of pregnancy:
- 280 days / 40 weeks from last normal menstrual period (LNMP)
- 266 days / 38 weeks from fertilization
8. Monthly Changes (High-Yield Table)
| Month/Stage | Key Development |
|---|---|
| 3rd month | Head = 1/2 of CRL; face becomes human-looking; external genitalia visible by week 12 (sex determinable on ultrasound) |
| 4th-5th months | Rapid lengthening; CRL ~15 cm (half of newborn total length); lanugo hair appears; eyebrows and head hair visible |
| 5th month | Mother can feel fetal movements (quickening); first spontaneous movements start at 7.5 weeks |
| 6th month | Skin is reddish and wrinkled (lack of subcutaneous connective tissue); CRL ~25 cm, weight ~1,100 g by 6.5-7 months; 90% survival chance if born at this stage |
| Last 2 months (8th-9th) | Well-rounded contours due to subcutaneous fat deposition; skin covered by vernix caseosa |
9. Important Survival & Birth Facts
- Fetus born early in 6th month has great difficulty surviving - respiratory and CNS not sufficiently differentiated
- By 6.5-7 months: 90% survival chance
- High neonatal mortality in:
- Low birth weight (LBW): ≤ 2500 g
- Very low birth weight (VLBW): ≤ 1500 g
At full-term birth:
| Parameter | Value |
|---|---|
| Weight | 3,000 - 3,400 g |
| CRL | ~36 cm |
| CHL | ~50 cm |
| Head proportion | 1/4 of CHL |
10. Special Structures
- Lanugo hair - fine hair covering the fetus in 4th-5th months
- Vernix caseosa - whitish, fatty substance from sebaceous glands covering the skin at end of intrauterine life; acts as a natural moisturizer and barrier against amniotic fluid
- Skull circumference - largest circumference at end of 9th month; important for passage through birth canal
Quick-Recall Cheat Sheet
| Topic | Key Fact |
|---|---|
| Placenta weight | ~500 g |
| Placenta size | 3 cm thick, 20 cm diameter |
| hCG source | Syncytiotrophoblast |
| hCG role | Basis of pregnancy tests |
| Fetal period start | Week 9 |
| Pregnancy duration | 40 weeks from LNMP / 38 weeks from fertilization |
| Sex determinable | By week 12 (ultrasound) |
| Quickening | 5th month (mother feels movements) |
| Survival at 6.5-7 months | 90% chance |
| Birth weight (normal) | 3,000-3,400 g |
| Vernix caseosa | Sebaceous gland secretion, protects from amniotic fluid |
| Lanugo | Fine fetal hair (4th-5th months) |
| Head at birth | 1/4 of CHL |
Sources: Sadler - Langman's Medical Embryology (14th ed.) and Carlson - Human Embryology and Developmental Biology (5th ed.), as cited in the lecture slides.
Give me MCQ type questions from this pdf
Here are 20 MCQ-type exam questions based on the Embryology Lecture 11 - Fetus and Placenta PDF:
MCQ Practice Questions
Embryology L5 - Fetus & Placenta
Q1. The placenta is best described as which type of organ?
- A) Purely fetal organ
- B) Purely maternal organ
- C) Fetomaternal organ ✅
- D) Extraembryonic organ only
Q2. The fetal component of the placenta is derived from:
- A) Uterine endometrium
- B) Trophoblast and extraembryonic mesoderm ✅
- C) Decidua basalis only
- D) Uterine myometrium
Q3. By the beginning of the 2nd month, the trophoblast is characterized by:
- A) Primary villi only
- B) A smooth, avascular surface
- C) A great number of secondary and tertiary villi giving it a radial appearance ✅
- D) Complete disappearance of cytotrophoblast
Q4. Maternal blood reaches the intervillous spaces through:
- A) Endometrial veins
- B) Spiral arteries ✅
- C) Umbilical arteries
- D) Chorionic capillaries
Q5. By what month does the cytotrophoblast essentially disappear?
- A) 3rd month
- B) 4th month
- C) 5th month ✅
- D) 6th month
Q6. Which decidual tissue lies between the chorionic vesicle and the uterine myometrium at the implantation site?
- A) Decidua capsularis
- B) Decidua parietalis
- C) Decidua basalis ✅
- D) Decidua functionalis
Q7. Chorion frondosum refers to:
- A) The smooth part of the chorion that atrophies
- B) The flourishing chorionic villi that ultimately become the placenta ✅
- C) The decidua overlying the embryo
- D) The extraembryonic mesoderm
Q8. What happens to the decidua capsularis by midpregnancy?
- A) It becomes the chorion frondosum
- B) It fuses with the decidua parietalis ✅
- C) It forms the umbilical cord
- D) It differentiates into syncytiotrophoblast
Q9. The mature placenta weighs approximately:
- A) 200 g
- B) 350 g
- C) 500 g ✅
- D) 750 g
Q10. Which of the following substances moves from FETUS to MOTHER across the placenta?
- A) Oxygen
- B) Nutrients
- C) Hormones from mother
- D) Carbon dioxide and urea ✅
Q11. Human chorionic gonadotropin (hCG) is produced by the:
- A) Cytotrophoblast
- B) Decidua basalis
- C) Syncytiotrophoblast ✅
- D) Chorionic mesoderm
Q12. The presence of hCG in maternal urine is the basis for:
- A) Blood typing in pregnancy
- B) Common pregnancy tests ✅
- C) Detecting fetal chromosomal abnormalities
- D) Estimating gestational age
Q13. The fetal period begins at:
- A) Week 4
- B) Week 6
- C) Week 9 ✅
- D) Week 12
Q14. Crown-Rump Length (CRL) represents:
- A) Standing height
- B) Sitting height ✅
- C) Head circumference
- D) Length from crown to toe
Q15. The standard length of pregnancy calculated from the Last Normal Menstrual Period (LNMP) is:
- A) 266 days / 38 weeks
- B) 270 days / 39 weeks
- C) 280 days / 40 weeks ✅
- D) 290 days / 41 weeks
Q16. By the 12th week of development, which of the following can be determined by ultrasound?
- A) Blood group of the fetus
- B) Presence of lanugo hair
- C) Sex of the fetus ✅
- D) Vernix caseosa formation
Q17. At what month do fetal movements first become felt by the mother?
- A) 3rd month
- B) 4th month
- C) 5th month ✅
- D) 6th month
Q18. A fetus born at 6.5 to 7 months has approximately what survival rate?
- A) 50%
- B) 70%
- C) 80%
- D) 90% ✅
Q19. Vernix caseosa is composed of secretory products from which glands, and its primary role is:
- A) Sweat glands; regulation of body temperature
- B) Sebaceous glands; protection from amniotic fluid ✅
- C) Mucous glands; lubrication of the birth canal
- D) Lacrimal glands; moisturizing the skin
Q20. At the time of full-term birth, the normal fetal weight and Crown-Heel Length (CHL) are:
- A) 2,500 g and 40 cm
- B) 3,000-3,400 g and 50 cm ✅
- C) 3,500-4,000 g and 55 cm
- D) 2,800 g and 45 cm
Answer Key
| Q | Ans | Q | Ans |
|---|---|---|---|
| 1 | C | 11 | C |
| 2 | B | 12 | B |
| 3 | C | 13 | C |
| 4 | B | 14 | B |
| 5 | C | 15 | C |
| 6 | C | 16 | C |
| 7 | B | 17 | C |
| 8 | B | 18 | D |
| 9 | C | 19 | B |
| 10 | D | 20 | B |
Tip for exam: Pay special attention to the numbers (placenta dimensions, birth weight, gestational ages) and the decidua/chorion terminology - these are classic MCQ targets!
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