Antral follicles are small follicles (about 2-8 mm in diameter) that we can see – and measure and count – with ultrasound. Antral follicles are also referred to as resting follicles.
Vaginal ultrasound is the best way to accurately assess and count these small structures. In my opinion, the antral follicle counts (in conjunction with female age) are by far the best tool that we currently have for estimating ovarian reserve and/or chances for pregnancy with in vitro fertilization.
As women age, they have less eggs (primordial follicles) remaining, therefore they have fewer antral follicles on ultrasound.
- When there are an average (or high) number of antral follicles, we tend to get a “good” response with many mature follicles. We tend to get a good number of eggs at retrieval in these cases. Pregnancy rates are higher than average.
- When there are few antral follicles, we tend to get a poor response with few mature follicles. Cancellation of the IVF cycle is much more common when there is a low antral count. Pregnancy rates are lower overall in this group. The reduction in success rates is more pronounced in women over 35 years old.
- When the number of antral follicles is intermediate, the response is not as predictable. In most cases the response is intermediate. However, we could also have either a low or a good response when the antral counts are intermediate. Pregnancy rates are pretty good overall in this group.
Ultrasound image of an ovary at the beginning of a menstrual cycle. No medications are being given. The ovary is outlined in blue. There are numerous antral follicles visible – marked with red spots. 16 are seen in this image, this ovary had a total of 35 antrals (only 1 plane is shown above) This is a polycystic ovary, with a high antral count and volume (ovary = 37 by 19.5mm) This woman had irregular periods and was a “high responder” to injectable FSH medication.
Ultrasound image of an ovary at the beginning of a menstrual cycle. No medications are being given. The ovary is outlined in blue. 9 antral follicles are seen – marked with red spots The ovary has normal volume (cursors measuring ovary = 30 by 17.8mm) This woman had regular periods and a normal response to injectable FSH drugs
An ovary is outlined in blue and is small (low volume) with only 1 antral follicle. Her other ovary had only 2 antral follicles. She had regular periods and a normal day 3 FSH test. Attempts to stimulate her ovaries for IVF were not successful