Follicle Monitoring

Follicle monitoring (follicular tracking) is a series of transvaginal ultrasound scans performed during a menstrual cycle to track follicle growth, determine ovulation timing, and assess the endometrium. It is essential for: (1) Natural cycle TTC — confirms ovulation occurred; (2) IUI — times insemination to within 24–36h of ovulation; (3) Ovulation induction — guides FSH/letrozole dose and trigger shot timing; (4) IVF — determines exact trigger shot timing and egg retrieval schedule.

In IVF/IUI cycles, the trigger shot (hCG or GnRH agonist) is given when the leading follicle reaches ≥18mm — with most protocols waiting for ≥2 follicles at ≥17–18mm in IVF. In OI/IUI cycles: trigger when the lead follicle is ≥18–20mm and endometrium is ≥8mm trilaminar. A follicle <16mm at trigger yields immature eggs; a follicle >24–25mm at trigger may have ovulated prematurely or yields post-mature eggs. Egg retrieval is performed 35–36 hours after trigger.

Natural cycle monitoring: 2–3 scans per cycle (Day 9–10 start, then every 1–2 days until ovulation confirmed). OI/IUI stimulated cycle: 3–4 scans (Day 2 baseline + Day 8–10 first check + 1–2 follow-up scans before trigger). IVF cycle: 4–5 scans (Day 2–4 baseline → Day 5–6 → Day 7–8 → Day 9–11 trigger decision). Blood tests (E2, sometimes LH) are drawn concurrently with each monitoring scan.

Failure to develop a dominant follicle (no follicle reaching ≥10mm by Day 12–14) indicates anovulation — the cycle will not result in ovulation or natural conception. Causes: PCOS (multiple small follicles without dominant development), poor ovarian response (low AFC, high FSH), hypothyroidism, hyperprolactinaemia, or excessive stimulation in a poor responder. Management: increase gonadotropin dose; switch ovulation induction agent; investigate hormonal cause; consider poor responder IVF protocol.

Yes — TVS can confirm ovulation by showing: (1) follicle collapse (disappears or becomes irregular/smaller); (2) new free fluid in the pouch of Douglas (follicular fluid released at ovulation); (3) endometrial pattern change (from trilaminar to homogeneous echogenic). These signs appear 0–24 hours after ovulation. Absence of these signs with a persistent mature follicle may indicate luteinised unruptured follicle syndrome (LUFS) — follicle luteinises without physically releasing the egg.