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AN78.1-5 | Second week of development — Self-Directed Learning
CLINICAL SCENARIO
A 26-year-old woman presents to MGMCRI Emergency at 7 weeks with severe vaginal bleeding and cramping. Her urine pregnancy test is positive. Transvaginal ultrasound shows no intrauterine gestational sac. Her beta-hCG is 3,200 IU/L. The on-call resident suspects ectopic pregnancy. But her husband asks: 'What does it mean that the baby hasn't reached the uterus? Where is it? Why does this happen?' — The answer requires understanding the second week of development: implantation, trophoblast invasion, and what can go wrong.
WHY THIS MATTERS
The second week — sometimes called 'week of twos' — establishes the fundamental architecture of the embryo and the placenta. Understanding implantation explains: (1) the basis of the pregnancy test (hCG from syncytiotrophoblast); (2) ectopic pregnancy (abnormal implantation); (3) placenta praevia and placenta accreta (abnormal sites and depth of implantation); (4) early pregnancy loss (implantation failure accounts for ~75% of all conceptus loss). The beta-hCG doubling time is used clinically to distinguish normal intrauterine from abnormal (ectopic/failing) pregnancies.
RECALL
Recall from Week 1 development:
• What is a blastocyst? Name its three components (inner cell mass, trophoblast, blastocoele).
• When does the zona pellucida disappear?
• What is the difference between the embryonic and abembryonic poles?
Implantation — Timing, Site, and Mechanism
Timing: Day 6–10 post-fertilisation (approximately day 20–24 of a 28-day cycle)
Normal site: Posterior wall of the body of uterus, superior part — endometrium in secretory (luteal) phase
Prerequisites for implantation:
1. Zona pellucida hatching (day 5–6): blastocyst sheds zona → exposes trophoblast
2. Endometrial receptivity: progesterone from corpus luteum prepares endometrium (window of implantation: days 20–24)
3. Molecular dialogue: L-selectin, trophinin, integrins, heparin-binding EGF
Stages of Implantation
1. Apposition: blastocyst loosely contacts endometrium (embryonic pole towards endometrium)
2. Adhesion: trophoblast cells attach firmly to endometrial epithelium
3. Invasion (day 7 onwards): syncytiotrophoblast invades endometrial stroma, erodes maternal capillaries
Trophoblast Differentiation (Day 8–9)
- Inner layer: Cytotrophoblast (cellular, mitotically active, mononuclear) — stem cell layer
- Outer layer: Syncytiotrophoblast (multinucleated syncytium formed by fusion of cytotrophoblast cells; NO mitosis in syncytium)
- Syncytiotrophoblast invades endometrium aggressively — produces hCG (human chorionic gonadotrophin)
- hCG: maintains corpus luteum → maintains progesterone secretion → prevents menstruation; basis of pregnancy test
- hCG peaks at weeks 8–10, then falls (placenta takes over progesterone production)
Lacunae and Uteroplacental Circulation (Day 9–13)
- Syncytiotrophoblast erodes maternal capillaries → blood fills lacunar spaces
- Trophoblast columns grow between lacunae → primary stem villi (precursors of chorionic villi)
- Maternal blood circulates through lacunae → primitive uteroplacental circulation established
Bilaminar Embryonic Disc
Formation of the bilaminar disc (Day 8–14)
While trophoblast is invading the endometrium, the inner cell mass reorganises:
- Inner cell mass → embryoblast → differentiates into TWO layers:
1. Epiblast (upper, columnar cells, adjacent to amniotic cavity) — gives rise to ALL three germ layers + amnion
2. Hypoblast (lower, cuboidal cells, adjacent to blastocoele) — gives rise to extraembryonic endoderm (yolk sac wall), NOT to embryo proper
Amniotic cavity (Day 8): Forms within epiblast as a fluid-filled space
Primary yolk sac (Day 8): Hypoblast cells line blastocoele → primary yolk sac (exocoelomic membrane)
Secondary yolk sac (Day 13): Primary yolk sac pinches off → secondary (definitive) yolk sac (smaller)
Extraembryonic Mesoderm (derived from hypoblast/trophoblast cells, Day 12–13):
Fills space between trophoblast and exocoelomic membrane
- Extraembryonic coelom (chorionic cavity): splits extraembryonic mesoderm into splanchnic (inner) and somatic (outer) layers
- Somatic layer + trophoblast → chorion
- Splanchnic layer + yolk sac endoderm → yolk sac wall
Connecting stalk (future umbilical cord): attaches embryo to trophoblast at abembryonic pole
'Week of Twos' Summary
- 2 trophoblast layers: cytotrophoblast + syncytiotrophoblast
- 2 embryonic disc layers: epiblast + hypoblast (bilaminar disc)
- 2 cavities: amniotic cavity + yolk sac
- 2 extraembryonic mesoderm layers: somatic + splanchnic
- 2 key hormones: hCG (trophoblast) + progesterone (corpus luteum, maintained by hCG)
SELF-CHECK
A. Normal early intrauterine pregnancy — too early to see sac
B. Ectopic pregnancy — hCG not doubling is the key finding
C. Complete molar pregnancy
D. Normal — hCG level correlates with gestational age
Reveal Answer
Answer: A.
CLINICAL PEARL
Implantation Abnormalities — Indian Clinical Context
- Ectopic pregnancy: In India, PID from Chlamydia/Gonorrhoea (often untreated) causes tubal scarring — the leading cause of ectopic pregnancy. After one ectopic, risk of recurrence is 10–25%. After two ectopics, risk is 50%.
- Placenta praevia: Low implantation (over or near cervical os); risk factors include prior CS (uterine scar), multiparity, multiple gestation. In India, grand multiparity is a significant risk factor in rural populations.
- Placenta accreta spectrum: Abnormally invasive trophoblast (accreta/increta/percreta) — dramatically increasing due to rising CS rates. India has one of the highest CS rates globally (21.5% nationally, >50% in private hospitals). Accreta in a uterine scar is a major cause of peripartum hysterectomy and maternal mortality.
REFLECT
KEY TAKEAWAYS
Core Take-Aways
- Implantation: Day 6–10; posterior uterine wall; requires zona hatching + endometrial receptivity
- Trophoblast differentiation: cytotrophoblast (inner, mitotic) → syncytiotrophoblast (outer, invasive, hCG-producing)
- hCG: maintains corpus luteum → progesterone → no menstruation; basis of pregnancy test; peaks week 8–10
- Bilaminar disc: epiblast (future embryo + amnion) + hypoblast (extraembryonic structures only)
- 'Week of twos': 2 trophoblast layers, 2 disc layers, 2 cavities, 2 extraembryonic mesoderm layers
- Implantation disorders: ectopic (PID scar), placenta praevia (low implantation), placenta accreta (rising with CS rates in India)
- Clinical signs of ectopic: non-doubling beta-hCG + empty uterus on TVS = emergency