AN78.1-5 | Second week of development — Summary & Reflection

REFLECT

KEY TAKEAWAYS

Key Takeaways — Second Week of Development

Implantation (AN78.3):
- Normal site: posterior wall of uterine fundus
- Three phases: apposition → adhesion → invasion (Days 6–12)
- Implantation window: Days 20–24 of menstrual cycle
- Ectopic pregnancy: 95% tubal (ampulla most common); tubal rupture is a surgical emergency
- Placenta praevia: low implantation near internal os → third-trimester bleeding

Trophoblast (AN78.2):
- Cytotrophoblast (inner): divides, feeds cells to the outer layer
- Syncytiotrophoblast (outer): invades, does NOT divide, produces hCG, forms lacunae
- Lacunae → primitive uteroplacental circulation (haemochorial placenta)
- hCG rescues the corpus luteum → maintains progesterone → maintains pregnancy

Bilaminar Disc & Blastocyst Events (AN78.1):
- Embryoblast → epiblast (dorsal, all germ layers) + hypoblast (ventral, extraembryonic)
- Amniotic cavity from epiblast; primary → secondary yolk sac from hypoblast
- Prochordal plate = cranial end marker, site of future mouth

Extraembryonic Mesoderm (AN78.4):
- Forms by Day 12, splits into somatic (lines trophoblast → chorion) and splanchnic (covers yolk sac)
- Extraembryonic coelom = chorionic cavity
- Connecting stalk = future umbilical cord
- Two body axes established: dorsoventral + craniocaudal

Clinical Correlates (AN78.5):
- Decidual reaction: stromal cells → decidual cells (glycogen + lipid storage)
- Decidua basalis (under embryo → placenta), capsularis (over embryo), parietalis (rest)
- hCG detectable in blood by Day 8–10, urine by Day ~14 post-fertilisation
- Doubling time of hCG: every 48–72 hours (slower rise → suspect ectopic)
- ~50–60% of fertilised eggs fail to implant → most pregnancy losses are subclinical

The "Week of Twos": two trophoblast layers, two embryonic disc layers, two cavities, two extraembryonic mesoderm layers, two nutrition modes.