Last-minute Review: Female Reproductive Anatomy & Menstrual Cycle

Embryology and initial anatomy

  • Sex determination vs sex differentiation: determination sets XX (female) or XY (male) at fertilization; Weeks 1–6 of gestation are undifferentiated for external genitalia.
  • Germ cell migration: germ cells migrate to genital ridges by Week 6; gonadal cords form the primitive gonads, later becoming testes or ovaries.
  • Internal ducts: two pairs present in all embryos – Wolffian ducts and Müllerian ducts.
    • Male: Wolffian ducts develop into epididymis, vas deferens, and seminal vesicles.
    • Female: Müllerian ducts develop into fallopian tubes, uterus, cervix, and upper vagina.
  • Common anomaly: bicornuate uterus from incomplete Müllerian duct fusion; potential for two uterine horns opening to one vagina; pregnancy risk and growth considerations.

External and internal female anatomy (brief)

  • External genitalia include the vulva; Bartholin (Bartholin’s) glands are at the lower vestibule and secrete lubrication; infection may require antibiotics or drainage.
  • Uterus: fundus, body, cervix; typical position is anteverted/anteflexed; three tissue layers: peritoneum/parametrium, myometrium, endometrium.
  • Endometrium has basalis (basalis layer that does not shed) and functionalis (shed during menstruation; proliferates under estrogen and secretory under progesterone).
  • Ovaries: cortex and medulla; surface germinal epithelium; primary ovarian function is to produce eggs and sex steroids.
  • Fallopian tubes: mucosa lined by ciliated cells; fimbriae assist oocyte capture; fertilization typically occurs in the ampulla.

Embryology and clinical relevance of ducts

  • Müllerian structures form female internal genitalia; Wolffian structures regress in typical female development.
  • Clinical relevance: tubal ectopic pregnancy risk when implantation occurs outside the uterus; tubal adhesions can impede transport of gametes.

Uterine and endometrial architecture (key points)

  • Uterine tissue layers: peritoneum/parametrium, myometrium (thick smooth muscle), endometrium (functional layer sheds in menses; basalis remains).
  • Endometrium proliferates under estrogen, then secretory changes under progesterone to support implantation.
  • Endometriosis: ectopic endometrial tissue outside uterus; responds to ovarian hormones; causes pain and dysmenorrhea.

Menstrual cycle overview (two main phases)

  • Typical cycle length: 2828 days on average; normal range: 2323–3535 days.
  • Day 1 = first day of menstrual bleeding; menstruation lasts 44–77 days (average bleed ~3030 mL, often with clots).
  • Phases:
    • Follicular phase (estrogen-dominant): follicle maturation; estradiol rises; endometrium proliferates.
    • Ovulation: mid-cycle, around day 1414 in a typical 2828-day cycle; LH surge triggers ovulation; mature follicle releases oocyte.
    • Luteal (luteal/progestational) phase: corpus luteum forms and secretes estradiol and progesterone; endometrium becomes secretory and ready for implantation.
  • Corpus luteum lasts about 1414 days unless pregnancy occurs; if pregnancy does occur, it is maintained by hCG until placenta takes over.
  • If no fertilization, progesterone falls, endometrium sheds, and a new cycle starts (menses around days 00–55 of the new cycle).
  • Fertile window: approximately five days before ovulation to one day after ovulation; sperm can survive up to about five days in the female tract.
  • Ovulation tests detect the LH surge (not a binary “on/off” LH; there is always some LH present).
  • Basal body temperature may rise slightly after ovulation, reflecting the luteal phase.

Hormonal orchestration (GnRH–Gonad axis)

  • Hypothalamus releases GnRH in pulsatile fashion → anterior pituitary secretes FSH and LH.
  • Follicular phase: FSH promotes follicle growth; rising estradiol builds the endometrium.
  • Ovulation: LH surge triggered by high estradiol leads to ovulation; progesterone rises after ovulation.
  • Luteal phase: corpus luteum secretes estradiol and progesterone; progesterone supports endometrium and provides negative feedback on FSH/LH.
  • End of cycle: if no pregnancy, corpus luteum regresses, progesterone and estrogen fall, menses begins.

Contraception and clinical management basics

  • Hormonal contraceptives suppress ovulation and thin the endometrium; consistent daily timing enhances effectiveness.
  • Placebo week in birth control packs represents withdrawal bleed, not a true period.
  • Dysmenorrhea management:
    • Primary dysmenorrhea: due to prostaglandins; NSAIDs reduce prostaglandin synthesis and pain.
    • Hormonal contraceptives may also help by suppressing ovulation and endometrial proliferation.
  • Anovulatory cycles:
    • Common in adolescence and perimenopause; no ovulation or corpus luteum formation; irregular menses can occur.
  • Amenorrhea definitions:
    • Primary: no menarche by age 1313 with no secondary sex characteristics, or by age 1515 with secondary sex characteristics but no menses.
    • Secondary: previously menstruated, then no menses for at least 66 months (some sources use 33 months).
  • Primary amenorrhea etiologies (high yield): Turner syndrome (gonadal dysgenesis), Müllerian agenesis (anatomical absence of uterus/vagina), imperforate hymen or transverse vaginal septum, and other anatomical or endocrine causes.
  • Secondary amenorrhea etiologies: pregnancy (most common), lactation/hyperprolactinemia, thyroid dysfunction, PCOS, hyperandrogenism, endometrial suppression (e.g., OCPs), and structural issues (Asherman syndrome).
  • Special notes on anatomy-related infertility: imperforate hymen, transverse septum, and cervical hypoplasia may require surgical correction; ovaries should be preserved when possible to maintain endogenous estrogen activity.
  • Menopause and later cycles: cycles become irregular as ovarian function wanes; this discussion leads into subsequent topics.

Quick clinical checklists (high-yield reminders)

  • Ovulation and cycle timing: LH surge precedes ovulation; day 14 is a reference point in a 28-day cycle; ovulation occurs about two weeks before menses; the egg is viable ~24 hours; fertile window ~5 days before to 1 day after ovulation.
  • Endometrium readiness: estrogen drives proliferation; progesterone drives secretory transformation; implantation requires a receptive endometrium.
  • Primary vs secondary amenorrhea: remember the age thresholds and need for evaluation when thresholds are not met.
  • Dysmenorrhea treatment: NSAIDs first-line; consider hormonal suppression if pain is disabling.
  • Ectopic pregnancy risk: tubal factors or adhesions may predispose to implantation outside the uterus; prompt evaluation is critical when pregnancy is suspected with unusual pain or bleeding.

Key definitions (recap)

  • Menarche: first menstrual period; puberty begins with breast development; followed by pubic/axillary hair growth and growth spurt.
  • Puberty: development of secondary sexual characteristics and reproductive capability.
  • Endometriosis: ectopic endometrial tissue outside the uterus, responsive to ovarian hormones.
  • Bartholin glands: lubrication glands at the posterior-lateral vaginal introitus; infection may require antibiotics or drainage.
  • Progestational (progesterone) phase: luteal-phase dominance preparing the endometrium for implantation.