Modulation of uterine artery resistance to blood flow by the oral contraceptive pill.
The changes in uterine artery resistance to blood flow were studied during a normal ovulatory cycle (control) and during a cycle on the combined oral contraceptive pill in 10 healthy women, aged 18-35 years, using transvaginal color Doppler imaging. Ovulation was monitored using ultrasound and hormonal assays during both cycles. The Pulsatility Index (PI) was used as a measure of uterine artery resistance, on days 8 (midproliferative) and 22 (midluteal) of the control cycle and on days 22 (maximal ovarian suppression) and 28 (minimal ovarian suppression) of the pill cycle. During the pill cycle, the uterine artery resistance decreased from a mean PI = 4.37 (range 2.4-7.95) on day 22 to a mean of 2.79 (1.94-4.99) on day 28, p = 0.006. The uterine artery resistance was significantly higher on day 22 during the pill cycle compared to the same day of the control cycle, p < 0.0001. Anovulatory cycles on the oral contraceptive pill are associated with an increase in uterine artery resistance and a decrease in uterine perfusion, this effect being reversed during the pill-free week.