In the first randomized trial, involving more than 600 women in 32 centres, researchers found that a patch delivering norelgestromin (150 µg) plus ethinyl estradiol (20 µg) suppressed follicular development in the ovary as effectively as pills containing oral norgestimate (250 µg) plus ethinyl estradiol (35 µg). Weekly ultrasound imaging was performed to measure the maximum mean follicular diameter, a reliable indicator of how close the egg sac is to releasing an egg.

A second trial, involving 136 women in 12 centres, showed that the patch is more effective than 3 different oral contraceptive pills in suppressing follicular development. It also showed that the patch is more forgiving after an intentional 3-day dosing error than either oral levonorgestrel (50/75/125 µg) plus ethinyl estradiol (30/40/30 µg) or oral levenorgestrel (100 µg) plus ethinyl estradiol (20 µg).

Dr. Roger Pierson, a professor of obstetrics and gynecology at the University of Saskatchewan who was lead researcher on the studies, said that while the research team was not surprised to learn that the patch performed well, members of his team had not expected that it would turn out to be so much better at suppressing the ovaries than the pill.

"The reason the patch appears to work so well is because it delivers a constant level of hormone," said Pierson. "This is very different from the pill, where the hormone level in your blood is raised as the pills are metabolized, then disappears. You get a peak-and-valley effect over each day that the pill is taken."

The other big plus, said Pierson, is that the patch seems to be more forgiving than oral contraceptives. "The price that you pay for a mistake is not necessarily as dire as it might be."