@gemiwing: Progesterone and testosterone are hormones. Also, estrogen peaks right around ovulation, so it makes a lot of sense that this would be when a singer’s voice would be highest…interesting study.
Here’s another study (or who knows, maybe the same one) finding basically the same thing:
At premenstruation and the onset of menstruation, the magnitude of frequency perturbation was not notably different from the average behavior of individual subjects. The time of ovulation was associated with a notable change in the magnitude of frequency perturbation for most subjects. It was suggested that ovarian hormone fluctuations alter neurotransmitter levels, which results in changes in the motor and sensory processes involved in laryngeal control.
from the Journal of Voice.
One other thought…do you take hormonal contraception, or would you consider doing so to smooth out your hormone levels?