A sensor for indirect estimation of breast milk intake by an infant, the sensor comprising: a housing configured to be externally attached to an incontinence product worn by a subject, said housing comprising: a light-emitting diode (LED) configured to illuminate the incontinence product; a photodetector configured to output an indication of the amount of light reflected from the incontinence product; and an integrated circuit configured to: (a) receive the indication from said photodetector, (b) compute, based on the indication, an amount of urine secreted by the subject into the incontinence product, and (c) estimate the amount of breast milk intake by the infant based on the amount of urine secreted by the subject into the incontinence product.