FemWear: A Parameter-Efficient Wearable Foundation Model for Women's Health

General wearable foundation models are pretrained across broad sensor streams and populations, but are not designed around women's-health tasks. We introduce FemWear, a specialized wearable foundation model that parameter-efficiently repurposes a pretrained multimodal wearable backbone. FemWear retains the patch projection and Transformer encoder, training 239,236 parameters (1.11% of a 21.54M-parameter encoder) through low-rank residual adapters and causal task-family heads. It learns one shared longitudinal representation for menstrual, symptom, affective, sleep/recovery, autonomic, activity, and pregnancy-related outcomes. We evaluate six cohorts with 63 comparable primary metrics, including 33 from women's-health cohorts, while retaining the 32-task OpenMHC ability-retention benchmark. On a fixed participant split over three seeds, FemWear improved cycle-phase macro-F1 by 8.15% and reduced mean absolute error for cramps, mood symptoms, and sleep problems by 9.32%, 5.80%, and 9.43%, respectively. In a stricter 42-participant nested leave-one-participant-out audit, 24-hour onset, 72-hour onset, and cramps retained positive changes of 2.87%, 6.35%, and 2.19%; phase, mood, and sleep were neutral or negative, and no endpoint had a strictly positive corrected confidence interval. Capacity-matched experiments outperformed a latest-day multilayer perceptron but not shared-GRU or multi-gate mixture-of-experts baselines. Train-only calibration reduced onset expected calibration error by 84.2--88.2% with zero temporal-nesting violations. FemWear enables targeted transfer and coherent probability outputs for women's-health research, but does not establish universal performance dominance or clinical validity.