OG3.1 | Ovulation, Menstruation, Fertilization and Implantation — Summary & Reflection

KEY TAKEAWAYS

Ovulation, Menstruation, Fertilisation and Implantation — Key Points:

• Folliculogenesis: primordial follicle pool (~1–2 million at birth, ~400,000 at puberty) recruited under FSH; dominant Graafian follicle selected; LH surge (triggered by peak oestradiol) causes follicular rupture ~36 hours later.
• Ovulation: secondary oocyte (arrested at metaphase II) + cumulus released; swept into ampulla of fallopian tube; viable for ~12–24 hours post-ovulation.
• Corpus luteum: forms from ruptured follicle; secretes progesterone + oestradiol for ~14 days; rescued by hCG from implanting trophoblast if conception occurs.
• Menstrual cycle phases: follicular (variable length, oestrogen-driven endometrial proliferation) → ovulation → luteal (fixed ~14 days, progesterone-driven secretory change and implantation window days 20–24).
• Oogenesis vs spermatogenesis: oogenesis begins fetal life, arrested at prophase I until LH surge resumes meiosis I, and at metaphase II until sperm penetration completes meiosis II; produces 1 ovum from 1 primary oocyte. Spermatogenesis from puberty, continuous, produces 4 spermatozoa per primary spermatocyte.
• Fertilisation: in the ampulla within ~12–24 h of ovulation; requires capacitation; acrosome reaction digests zona pellucida; cortical reaction prevents polyspermy; sperm penetration completes oocyte meiosis II; pronuclei fuse at first cleavage → zygote (2n=46).
• Implantation: zygote → morula → blastocyst (day 5); hatches from zona pellucida; adheres to posterior uterine fundus; syncytiotrophoblast invades endometrium and secretes hCG from ~day 8–9 post-fertilisation (basis of pregnancy test); decidualisation follows.
• Clinical points: luteal phase is fixed (~14 days); follicular phase varies (hence cycle length varies); hormonal contraceptives suppress LH surge; hCG doubles every ~48 hours in normal early pregnancy.

REFLECT

Consider the couple from the opening scenario — regular cycles, biphasic basal body temperature, normal semen analysis, yet 18 months of infertility. Having worked through this module, you now understand at which specific steps conception could be failing even when ovulation and sperm production appear normal: sperm capacitation, acrosome reaction, fertilisation timing (if intercourse is badly timed within the cycle), tubal transport, the implantation window, or trophoblast invasion. Kolb's cycle prompts you to connect this conceptual knowledge with clinical action: what history and investigations would you now request to narrow the failure point? What would a normal mid-luteal progesterone, a normal HyCoSy, and a normal hysteroscopy each tell you — and what would they leave unresolved? Write your reasoning down before your next infertility outpatient session.