Biology • Year 12 • Module 5 • Lesson 4
Mammalian Reproduction — Fertilisation, Implantation, Pregnancy and Birth
Apply the lesson framework to a real pregnancy hormone profile, an IVF clinic timeline and a real-world case from obstetric practice.
1. Sequence the steps — fertilisation to birth in a human pregnancy
The events below describe the journey of a single human pregnancy from ovulation through to labour. They are shuffled. Write the correct order (1–8) in the left-hand box beside each event. 8 marks
| Order | Event |
|---|---|
| The placenta forms from both maternal and embryonic tissues and takes over hormonal support of the uterine lining. | |
| A haploid sperm fuses with a haploid egg in the upper oviduct, restoring the diploid chromosome number. | |
| The blastocyst embeds in the thickened endometrium and embryonic tissue begins to release hCG. | |
| A surge of LH triggers ovulation; the ovum is released from the ovary into the oviduct. | |
| The zygote undergoes repeated mitotic division (cleavage) while travelling down the oviduct toward the uterus. | |
| Oxytocin levels rise sharply, driving rhythmic uterine contractions that push the fetus through the cervix. | |
| By the end of the first week, a hollow ball of cells — the blastocyst — has formed. | |
| Major organs and the body plan are laid down (embryonic stage) and the developing offspring is later termed a fetus as growth and maturation continue. |
2. Interpret data — hormone levels across pregnancy
The table below shows approximate maternal plasma concentrations of four hormones at four points in a typical human pregnancy. Values are adapted from Tortora & Derrickson, Principles of Anatomy and Physiology (15th ed., 2017), Chapter 29. 7 marks
| Time point | hCG (mIU mL⁻¹) | Progesterone (ng mL⁻¹) | Oestrogen (ng mL⁻¹) | Oxytocin (pg mL⁻¹) |
|---|---|---|---|---|
| Week 2 (just after implantation) | 50 | 15 | 2 | 2 |
| Week 10 (end of 1st trimester) | 100 000 | 40 | 8 | 3 |
| Week 24 (mid 2nd trimester) | 12 000 | 110 | 16 | 5 |
| Week 38 (onset of labour) | 5 000 | 180 | 25 | 40 |
Approximate values, adapted from Tortora & Derrickson (2017), Principles of Anatomy and Physiology, 15th ed.
2.1 Describe the trend in hCG across the four time points and link this trend to the hormone's role in early pregnancy. 2 marks
2.2 Identify the hormone whose concentration changes most dramatically between Week 24 and Week 38, and explain its role in labour. 2 marks
2.3 Predict what would happen to the pregnancy if maternal progesterone levels suddenly dropped to 5 ng mL⁻¹ at Week 10. Justify your prediction in terms of placental and endometrial function. 3 marks
3. Interpret a real-data graph — hCG concentration over time
The figure below shows the typical maternal serum concentration of human chorionic gonadotrophin (hCG) measured from the day of implantation through the first 16 weeks of pregnancy. Curve adapted from Korevaar et al., 2015, European Journal of Endocrinology 172: 593–601 (Generation R cohort, n ≈ 8200 singleton pregnancies). 7 marks
Figure adapted from Korevaar et al. (2015), European Journal of Endocrinology 172, 593–601 — Generation R singleton pregnancy cohort.
3.1 Describe the overall shape of the hCG curve from week 0 to week 16. 2 marks
3.2 Estimate the maternal serum hCG concentration at week 4 and at week 10. 2 marks
3.3 Home pregnancy tests detect hCG in urine above a threshold of roughly 25 mIU mL⁻¹. Using the curve, explain why a test taken the day before the expected period (~14 days post-fertilisation) can produce a false negative, but a test taken one week later is almost always positive. 3 marks
4. Case study — failure of implantation in an IVF cycle
A 34-year-old patient at the Royal Hospital for Women, Sydney undergoes IVF. Eggs are harvested and successfully fertilised with her partner's sperm in the laboratory. Two healthy day-5 blastocysts are transferred into her uterus on Day 0 of the cycle. Serum hCG is measured on Day 14: it reads <5 mIU mL⁻¹, indicating that no implantation has occurred. Imaging shows the patient has a thin endometrium (4 mm) on the day of transfer compared with the recommended >7 mm. The treating clinician notes that her progesterone supplementation was started 24 hours later than the standard protocol.
In 5–7 sentences, explain — using lesson content on fertilisation, implantation and hormonal control — why this IVF cycle most likely failed despite successful in-vitro fertilisation. Refer specifically to (a) the role of the endometrium, (b) the role of progesterone, and (c) why the clinic measures hCG (rather than progesterone) to confirm pregnancy. 6 marks
Q1 — Correct order of human fertilisation-to-birth events
- A surge of LH triggers ovulation; the ovum is released from the ovary into the oviduct.
- A haploid sperm fuses with a haploid egg in the upper oviduct, restoring the diploid chromosome number.
- The zygote undergoes repeated mitotic division (cleavage) while travelling down the oviduct toward the uterus.
- By the end of the first week, a hollow ball of cells — the blastocyst — has formed.
- The blastocyst embeds in the thickened endometrium and embryonic tissue begins to release hCG.
- The placenta forms from both maternal and embryonic tissues and takes over hormonal support of the uterine lining.
- Major organs and the body plan are laid down (embryonic stage) and the developing offspring is later termed a fetus as growth and maturation continue.
- Oxytocin levels rise sharply, driving rhythmic uterine contractions that push the fetus through the cervix.
Marking note: award 1 mark per correctly placed event.
Q2.1 — hCG trend and role (2 marks)
hCG rises sharply from 50 mIU mL⁻¹ just after implantation to a peak of ~100 000 mIU mL⁻¹ around week 10, then declines through the rest of pregnancy (12 000 at week 24, 5 000 at week 38) [1]. This pattern matches its biological role: hCG is produced by embryonic tissue after implantation and is needed mainly in early pregnancy to maintain progesterone support; once the placenta is fully established and produces its own progesterone, hCG is no longer required at peak levels [1].
Q2.2 — Largest change between week 24 and week 38 (2 marks)
Oxytocin shows the largest proportional rise — from 5 to 40 pg mL⁻¹, an eight-fold increase [1]. Its role in labour is to stimulate strong, rhythmic contractions of the smooth muscle of the uterus, pushing the fetus through the cervix; positive feedback amplifies this surge once labour begins [1].
Q2.3 — Prediction if progesterone fell to 5 ng mL⁻¹ at week 10 (3 marks)
The pregnancy would most likely fail (miscarriage) [1]. Progesterone is the dominant hormone maintaining the uterine lining (endometrium) so that it remains thickened and well vascularised to support the embryo [1]. A sudden drop to 5 ng mL⁻¹ — close to non-pregnant levels — would cause the endometrium to break down, severing the placental connection and ending the supply of oxygen and nutrients to the embryo [1]. (This is why early IVF pregnancies are routinely supported with exogenous progesterone.)
Q3.1 — Shape of the hCG curve (2 marks)
From week 0 to about week 2 hCG is essentially undetectable [0.5]. It then rises very steeply (roughly exponentially on the log axis) through weeks 2–10, reaching a peak of about 10⁵ mIU mL⁻¹ around week 10 [1]. Beyond week 10 it falls back toward ~10⁴ mIU mL⁻¹ by week 16 — a clear "rise-then-fall" curve with the peak in early pregnancy [0.5].
Q3.2 — Read-off values (2 marks)
Week 4 ≈ 5 000 mIU mL⁻¹ (10³·⁷) [1]. Week 10 ≈ 100 000 mIU mL⁻¹ (10⁵) — the peak [1]. Accept any answer within half a log unit (×3 or ÷3) of these values.
Q3.3 — False negative early, reliable later (3 marks)
Implantation only occurs around week 2 (≈ day 14 post-fertilisation), and hCG is only released by embryonic tissue after implantation [1]. On day 13 — the day before the expected period — hCG may still be below the home-test threshold of ~25 mIU mL⁻¹, producing a false negative [1]. One week later (around day 20), the curve shows hCG has risen by roughly two orders of magnitude into the thousands, well above any home-test threshold, so the test is almost always positive [1].
Q4 — IVF case study (6 marks, sample top-band response)
In this cycle, fertilisation itself was successful — the laboratory confirmed two healthy day-5 blastocysts. The failure must therefore have occurred at or after implantation. (a) The endometrium had to be thick and well-vascularised for the blastocyst to embed and establish exchange with maternal tissue; at only 4 mm (below the recommended 7 mm threshold) it could not support implantation. (b) Progesterone is the dominant hormone that thickens and maintains this endometrial lining after ovulation; a 24-hour delay in supplementation meant peak progesterone arrived after the implantation window had begun closing, so the endometrium never reached an implantation-ready state. (c) After implantation, the embryo releases hCG to keep progesterone support going; without implantation, no hCG is released, which is exactly why the day-14 hCG was <5 mIU mL⁻¹. Clinics measure hCG (not progesterone) because progesterone could be supplemented externally and would not prove an embryo had implanted — only hCG production confirms that embryonic tissue has embedded and is biologically active. The IVF cycle therefore failed not because of fertilisation but because of a failure of implantation, which is consistent with the lesson's point that fertilisation alone does not guarantee pregnancy.
Marking note: 1 mark for endometrial role, 1 mark for progesterone role, 1 mark for the timing/delay explanation, 1 mark for explaining why hCG (not progesterone) is the marker, 1 mark for tying it back to the fertilisation/implantation distinction, 1 mark for biological precision (uses terms blastocyst, endometrium, implantation, hCG correctly).