Shorter pill-free interval in combined oral contraceptives decreases follicular development.

The objective of the study was to determine the suppressive effect on ovarian activity of 20 micrograms ethinylestradiol plus 75 micrograms gestodene administered for 21 or 23 days. The study was designed as a double-blind, randomized, multicenter trial in 60 women. A pre-treatment cycle, three treatment cycles and a post-treatment period were monitored by ovarian ultrasound and by LH, FSH, 17 beta-estradiol and progesterone measurements every other day. No ovulation and no luteinized, unruptured follicle were observed. Suppression of ovarian activity was more pronounced by the 23-day regimen. 17 beta-Estradiol serum levels during the last six days of a cycle and during the first six days of the next cycle were significantly less (p < 0.05) in the 23-day regimen. The superiority of the 23-day regimen in comparison to the 21-day regimen with regard to the suppression of ovarian activity was shown in this study. The observed differences in the 17 beta-estradiol levels and follicular development between a 21-day and 23-day preparation combine to suggest that shortening the pill-free interval in combined oral contraceptives may increase the contraceptive safety margin in women on low-dose formulations.