Follicular recruitment: Does follicular recruitment occur in groups or continuously? A prospective observational study

Objective: The aim of this study was to investigate whether antral follicular recruitment in healthy reproductive-age women occurs intermittently in waves throughout the menstrual cycle or continuously. Methods: This single-center prospective observational study was conducted between August 2024 and April 2025. Twenty healthy volunteers aged 18–38 years with regular menstrual cycles were monitored for three months by transvaginal ultrasonography (TVUSG) performed every three days, resulting in a total of 598 examinations. Follicle counts were obtained using the AI-assisted Folliscan software and compared with manual measurements. A random intercept was defined for time-dependent total antral follicle counts, and the presence of regular cyclical patterns was tested using autocorrelation (ACF) and spectral analysis methods. In addition, linear, polynomial, natural cubic spline, and Fourier/sinusoidal functions were compared. An autoregressive correlation structure (AR(1)) was incorporated to model serial dependence, and model fit was assessed using Akaike Information Criterion (AIC), Bayesian Information Criterion (BIC), Root Mean Square Error (RMSE), marginal R2, and residual standard deviation (σ). Results: The participants' mean age was 27.9 ± 4.4 years, mean BMI was 22.7 ± 3.1 kg/m2, and median serum AMH level was 2.83 (2.04–3.68) ng/mL. Examination of the total antral follicle count (AFC) time series revealed no distinct periodic wave pattern other than short-term oscillations. The autocorrelation function (ACF) indicated a weak but short-term positive correlation between consecutive measurements, suggesting that follicle count fluctuations were related in the short term but independent and non-cyclical over longer intervals. Spectral analysis revealed no single dominant frequency peak. In linear mixed-effects models, time was modeled using linear, polynomial, natural cubic spline, and sinusoidal functions. The AR(1) + spline model, which achieved the lowest AIC (4293), BIC (4329), RMSE (8.67), and σ (8.85) values, demonstrated a slightly better fit than alternative models (marginal R2 = 0.07), yet was insufficient to show a structured pattern. These findings indicate that antral follicular development does not exhibit regular cyclical waves throughout 90 days but rather represents a continuous and stochastic process that varies among individuals and occurs at different time points.