How reproductive physiology is tested on Step 1
Tie each phase of the cycle to its dominant hormone.
| Hormone | Source | Key actions |
|---|---|---|
| FSH | Anterior pituitary | Follicle growth; aromatase in granulosa cells |
| LH | Anterior pituitary | Theca cell androgens; ovulation; corpus luteum |
| Estrogen | Ovary, placenta | Endometrial proliferation; LH surge; ↑ binding proteins (TBG, SHBG) |
| Progesterone | Corpus luteum, placenta | Secretory endometrium; maintains pregnancy; relaxes uterine muscle |
| hCG | Syncytiotrophoblast | Maintains corpus luteum; basis of pregnancy tests |
| Human placental lactogen | Placenta | Insulin resistance to supply glucose to the fetus |
Theca cells make androgens (under LH), which granulosa cells convert to estrogen (under FSH).
Physiologic changes of pregnancy
Many "abnormal" labs are normal in pregnancy.
- Blood: plasma volume rises more than red cell mass (dilutional anemia); clotting factors rise (hypercoagulable state).
- Heart and vessels: cardiac output rises about 30–50%; vascular resistance and blood pressure fall in the second trimester.
- Lungs: tidal volume and minute ventilation rise (progesterone) → mild respiratory alkalosis; functional residual capacity falls.
- Kidneys: GFR rises, so serum creatinine falls; glucose may appear in the urine.
- Endocrine: estrogen raises TBG, so total T4 rises with normal free T4; insulin resistance can unmask gestational diabetes.
Lactation and menopause
Prolactin and oxytocin drive breastfeeding; loss of ovarian follicles drives menopause.
- Lactation: prolactin makes milk once estrogen and progesterone fall after delivery; oxytocin causes milk let-down. Prolactin suppresses GnRH, delaying ovulation.
- Menopause: ovarian follicles run out, estrogen and inhibin fall, and FSH rises (the best lab marker). Hot flashes, vaginal atrophy, and bone loss follow.
Gestational trophoblastic disease
Abnormal fertilization produces a mole with very high hCG.
Worked example
Use the fixed luteal phase.
functions Worked example — timing ovulation
A woman with regular 35-day cycles wants to know when she ovulates. On which cycle day does ovulation most likely occur, and which hormone surge causes it?
- The luteal phase is fixed at about 14 days; the follicular phase varies.
- Ovulation therefore occurs about 14 days before the next menses.
- 35 − 14 = day 21.
- High estrogen from the dominant follicle triggers the LH surge that causes ovulation.
Answer. About day 21, after the LH surge.