4.1 Reproductive Anatomy, Puberty, the Menstrual Cycle & Müllerian Anomalies
Key Takeaways
The ovulatory cycle has a variable follicular phase and a relatively fixed luteal phase of about 14 days; ovulation follows the LH surge by roughly 24–36 hours.
Puberty in girls usually begins with thelarche around age 10, and menarche follows about 2–3 years later at an average age of about 12.4 years in the United States.
In adolescents, normal cycles last 21–45 days with menses of 7 days or less; ACOG calls the menstrual cycle a vital sign.
The septate uterus is the most common Müllerian anomaly and the one most linked to miscarriage; it is treated by hysteroscopic septum resection when indicated.
Müllerian anomalies often coexist with renal anomalies, so a uterine anomaly should prompt renal imaging.
Clinically Important Pelvic Anatomy
- Uterus: Fundus, body, isthmus (lower uterine segment in pregnancy), and cervix. Layers are the endometrium (functional and basal layers), myometrium, and serosa.
- Blood supply: The uterine artery arises from the anterior division of the internal iliac artery and crosses over the ureter about 1.5–2 cm lateral to the cervix ("water under the bridge"), the main site of ureteral injury at hysterectomy. The ovarian artery arises directly from the aorta and runs in the infundibulopelvic ligament.
- Ligaments: The round ligament runs from the uterine cornu through the inguinal canal to the labium majus (the source of round ligament pain in pregnancy). The cardinal and uterosacral ligaments provide apical support; their failure produces uterine or vault prolapse. The broad ligament is a peritoneal fold that carries the vessels.
- Ovary and tube: The fallopian tube has interstitial, isthmic, ampullary, and fimbrial segments; fertilization and most ectopic pregnancies occur in the ampulla.
- External genitalia: Bartholin glands lie at 4 and 8 o'clock in the vestibule; Skene glands are paraurethral.
The Hypothalamic-Pituitary-Ovarian Axis
- The hypothalamus releases GnRH in pulses. Fast pulses favor LH; slow pulses favor FSH. Continuous GnRH (as with GnRH agonist therapy) downregulates the pituitary.
- FSH recruits follicles and induces aromatase in granulosa cells; LH stimulates theca cells to make androgens, which granulosa cells convert to estradiol (the "two-cell, two-gonadotropin" model).
- Estradiol gives negative feedback at low levels but positive feedback when it stays high (about 200 pg/mL for about 50 hours), triggering the LH surge.
- Inhibin B (follicular phase) suppresses FSH; inhibin A and progesterone come from the corpus luteum.
The Ovarian and Endometrial Cycles
| Phase | Ovary | Endometrium | Hormones and Signs |
|---|---|---|---|
| Menses (days 1–5) | New follicular cohort recruited | Functional layer sheds | Low estradiol and progesterone; FSH rises |
| Follicular (variable length) | Dominant follicle selected | Proliferative: glands and stroma grow | Rising estradiol; clear, stretchy, abundant cervical mucus (spinnbarkeit, ferning) |
| Ovulation (about day 14 in a 28-day cycle) | Follicle ruptures about 24–36 hours after the LH surge begins | Early secretory changes | LH surge; mittelschmerz in some women |
| Luteal (about 14 days) | Corpus luteum secretes progesterone | Secretory: glycogen-rich glands, stromal edema | Progesterone raises basal body temperature 0.4–1.0 °F; thick cervical mucus; mid-luteal progesterone >3 ng/mL confirms ovulation |
Without pregnancy, the corpus luteum regresses after about 14 days, progesterone falls, and menses begin. With pregnancy, hCG rescues the corpus luteum until the placenta takes over progesterone production at about 7–10 weeks (the luteal-placental shift).
Puberty and Menarche
| Stage | Typical Age | Notes |
|---|---|---|
| Thelarche (breast budding, Tanner 2) | About 10 years (often earlier in Black girls) | Usually the first sign of puberty |
| Pubarche/adrenarche | Shortly after thelarche | Adrenal androgens drive pubic and axillary hair |
| Peak height velocity | Before menarche | Growth slows after menarche |
| Menarche | Average about 12.4 years in the United States | About 2–3 years after thelarche |
- Precocious puberty: Secondary sexual development before age 8 in girls warrants evaluation (bone age, LH/FSH, estradiol, and possibly a GnRH stimulation test and brain MRI).
- Delayed puberty: No breast development by age 13, or no menarche by age 15 or within 3 years of thelarche, triggers an amenorrhea evaluation.
- The adolescent cycle as a vital sign (ACOG): Cycles typically last 21–45 days, bleeding lasts 7 days or less, and changing a pad or tampon every 1–2 hours suggests heavy bleeding. Anovulatory cycles are common in the first 1–2 years after menarche, but bleeding that causes anemia needs evaluation for a bleeding disorder such as von Willebrand disease.
Menopause
Natural menopause occurs at an average age of about 51. Menopause before 40 is premature (primary ovarian insufficiency), and menopause from 40 to 45 is early; both increase lifetime cardiovascular and bone risk.
Müllerian (Paramesonephric) Anomalies
The paired Müllerian ducts form the fallopian tubes, uterus, cervix, and upper vagina; they must fuse and then resorb the midline septum. The lower vagina and hymen come from the urogenital sinus. Defects are classified by ASRM's 2021 Müllerian Anomalies Classification.
| Anomaly | Embryologic Error | Clinical Significance |
|---|---|---|
| Agenesis (MRKH syndrome) | Ducts fail to develop | Primary amenorrhea with normal breasts and pubic hair (46,XX); vaginal dilation therapy |
| Unicornuate uterus | One duct fails to develop | A rudimentary horn can bleed (cyclic pain) or host an ectopic pregnancy; strong renal agenesis link on the same side |
| Uterus didelphys | Complete failure of fusion | Two uteri and two cervices, often with a longitudinal vaginal septum; may cause obstructed hemivagina (OHVIRA) with ipsilateral renal agenesis |
| Bicornuate uterus | Partial failure of fusion | Indented fundal contour; malpresentation and preterm birth |
| Septate uterus | Failure of septum resorption | Most common anomaly and the one most associated with miscarriage; hysteroscopic resection is considered after recurrent loss |
| Arcuate uterus | Minimal residual septum | Usually a normal variant |
| Transverse vaginal septum | Failed fusion of Müllerian and sinus tissue | Cyclic pain, hematocolpos above a blind vagina |
| Imperforate hymen | Failed canalization of the urogenital sinus | Bulging bluish membrane at the introitus, cyclic pain, urinary retention; hymenotomy |
| DES-related (T-shaped) uterus | In utero diethylstilbestrol exposure | Preterm birth, ectopic pregnancy, vaginal clear cell adenocarcinoma |
Diagnosis: 3D transvaginal ultrasound or MRI best shows the external fundal contour (smooth or minimally indented in septate; deeply indented in bicornuate). Hysterosalpingography alone cannot reliably tell septate from bicornuate. Because the kidneys and Müllerian ducts develop together, renal ultrasound is recommended when a uterine anomaly is found.
A 28-year-old with regular 35-day cycles asks when she most likely ovulates. Assuming a typical luteal phase, on which cycle day does ovulation most likely occur?
Cycle day 14, because ovulation always occurs at midcycle
Cycle day 21, because the luteal phase is fixed at about 14 days
Cycle day 28, because the follicular phase is fixed at about 14 days
Cycle day 7, immediately after menses end
A 15-year-old reports menses every 24–30 days lasting 10 days, with heavy flow requiring a pad change every hour on the heaviest days since menarche at age 12. Her hemoglobin is 9.8 g/dL. Which evaluation is most appropriate?
Reassure her that anovulatory bleeding is normal in the first years after menarche and recheck in a year.
Order a pelvic MRI to rule out an obstructive Müllerian anomaly before any lab testing.
Screen for a bleeding disorder such as von Willebrand disease, with a CBC, ferritin, and pregnancy test.
Begin clomiphene citrate to regulate ovulation and recheck hemoglobin in 3 months.
A 30-year-old with two first-trimester miscarriages has a 3D ultrasound showing a smooth, normal external fundal contour with a thick fibrous tissue band dividing the upper endometrial cavity. Which statement is correct?
Septate uterus, the Müllerian anomaly most linked to miscarriage; hysteroscopic resection may be considered.
Bicornuate uterus, which requires abdominal metroplasty before any further pregnancy is attempted.
Uterus didelphys, so a second cervix and often a longitudinal vaginal septum should be visible.
A normal anatomic variant of the fundus that needs no further evaluation or counseling.
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