The size of ovarian follicles during the menstrual cycle is an important indicator of follicular development and can be especially important in fertility treatments such as timed intercourse, intrauterine insemination (IUI) and in vitro fertilization (IVF). Below is a general guide to the expected size of follicles at different stages of the menstrual cycle which is usually 28 – 30 day cycle.

D1 is the first day of bleeding in the menstrual cycle.

I. Follicular Phase of the menstrual cycle (Day 1-14)

1. Early Follicular Phase (Day 1-7):

  • Follicle Size: 3-8 mm
  • During the initial days of the menstrual cycle, multiple small antral follicles are present in the ovaries.

2. Mid-Follicular Phase (Day 8-12):

  • Follicle Size: 10-14 mm
  • Follicles start growing more rapidly in response to rising levels of follicle-stimulating hormone (FSH). By mid-cycle, a dominant follicle (one follicle which is choosen) usually emerges.

3. Late Follicular Phase (Day 13-14):

  • Follicle Size: 18-25 mm
  • The dominant follicle continues to grow and reach maturity. A mature (ripe) follicle is typically around 18-25 mm in diameter.

II. Ovulation (Day 14)

  • Follicle Size: 20-25 mm
  • Ovulation (release of egg or oocyte from the follicle) generally occurs when the dominant follicle reaches approximately 20-25 mm in size. The luteinizing hormone (LH) surge triggers the release of the egg from the mature follicle.

III. Luteal Phase (Post-Ovulation, Day 15-28)

  • After ovulation, the ruptured follicle transforms into the corpus luteum, which produces progesterone to support the pregnancy if it has happened.
  • Follicle size is no longer measured during the luteal phase as the focus shifts to the corpus luteum function.

IV. Monitoring Follicle Size During Fertility Treatments

  • In fertility treatments like follicular monitoring, IUI and IVF, careful monitoring of follicle size through ultrasound is essential to determine the optimal timing of intercourse, for triggering ovulation and subsequent insemination or egg retrieval.

1. Baseline Ultrasound (Day 2-4): An initial ultrasound is performed to assess baseline follicle size and count, as well as to check for ovarian cysts or other abnormalities. This is called baseline scan and done before starting any fertility treatment.

2. Day 5-12: Regular ultrasounds are conducted to monitor the follicle growth. The frequency of the scans may occur every alternate day or after every 2 days till ovulation.

3. Ultrasound to decide the trigger: When the dominant follicle reaches around 18-20 mm, a scan is done to decide the timing of the trigger for ovulation.

4. Scan to check for ovulation: Final scan done to check if ovulation ( in IUI cycles and timed cycles) has happened around 36 hours after the trigger is given or ovum pick up done in IVF cycles before 36 hours. 

Summary

  • Day 1-7: 3-8 mm
  • Day 8-12: 10-14 mm
  • Day 13-14: 18-25 mm (ready for ovulation)
  • Ovulation: 20-25 mm

These are general guidelines, but individual variations can occur according to the length of menstrual cycle. Regular monitoring by a fertility specialist ensures that the timing of ovulation or egg retrieval is optimised based on the specific growth patterns of the follicles in each cycle.