Follicular monitoring or follicular study is a vital component of in-vitro fertilization (IVF) assessment and timing. It basically employs a simple technique for assessing ovarian follicles at regular intervals and documenting the pathway to ovulation.
Transvaginal ultrasound is preferred and usually mandatory modality for monitoring follicles. Ultrasound monitoring may begin on day 3 of the cycle, to assess a baseline size, as well as exclude if any cyst remains from previous hyperstimulation or otherwise. It’s important to count the number of existing follicles, document two/three dimensions of each follicle, and also comment on shape (round/oval/rectangular/triangular), echogenicity (echogenic/hypoechoic/anechoic) and antral edges (smooth/intermediate/rough) if possible.
As the study progresses on day 7, we should start guessing the ovulatory dominant follicle i.e. dominant follicle which is destined to ovulate. Basically, there are three varieties of eligible follicles:
Atretic dominant follicle:
This follicle is usually largest follicle on day 3, but it is not destined to ovulate. It has an irregular shape, rough edges, and may be little echogenic.
Ovulatory dominant follicle:
This follicle is typically round, with smooth borders, and usually hypoechoic.
Anovulatory-luteinizing dominant follicle:
This dominant follicle grows at a good pace but fails to ovulate, and later becomes a cyst or luteinized. These are also round and smooth, however anechoic. This subtle recognition of echogenicity difference between hypoechoic and anechoic follicle can help determine whether a follicle is growing to ovulate.
Ovarian volume is easy to measure, although not a good predictor of IVF outcome. Now, it is documented, that a low ovarian volume does not always lead to anovulatory cycle. But, it is important to recognize a polycystic ovarian pattern and differentiate it from post-induction multicystic ovaries. Follicles arranged in the periphery forming a ‘necklace sign’, echogenic stroma, and more than 10 follicles of less than 9 mm in size, signify a polycystic pattern in induced cycle. While, follicles in the center as well as the periphery, are seen in normal induced multicystic ovaries.