Circadian variation in uterine artery blood flow indices during the follicular phase of the menstrual cycle.

We investigated for the presence of circadian variation in uterine artery blood flow indices during the late follicular phase of the menstrual cycle. Twenty women with regular menstrual cycles underwent transvaginal color Doppler assessment of blood flow in the uterine arteries on two occasions precisely 12 h apart on the same day in the follicular phase. Ultrasound examinations were performed between 06.00 and 10.00 and repeated between 18.00 and 22.00. Indices of uterine artery blood flow included the pulsatility index (PI) and the time-averaged maximum velocity (TAMX). The changes in PI and TAMX were related to variations in serum concentrations of estradiol and luteinizing hormone (LH). We noted a significantly higher PI and lower TAMX in the evening compared to the morning values, p < 0.001 and p = 0.01, respectively. There was no significant change in estradiol or LH concentrations between morning and evening (p > 0.05). There appears to be a circadian rhythm in uterine artery blood flow occurring independently of these hormonal parameters. The circadian changes in PI and TAMX may be larger than the previously described effects of hormones on these parameters. The accurate interpretation of uterine artery blood flow indices should take these time-related fluctuations into account.