Oral contraceptive pills are the most commonly utilized form of contraception in the United States. However, there are fears about the safety of combined oral contraceptive (COC) pills, particularly regarding the risks of venous thromboembolic events (VTE) among users. Progestin only pills (POP) have relatively few contraindications and are not associated with risk for VTE. However, because ovulation occurs in nearly half of all cycles in women using a POP, there is a belief that POPs are less effective in preventing pregnancy than a COC.  The only available POP in the United States is Norethindrone 0.35mg. This dose is one third of the progestin dose available in COCs. It is possible that the lower dose of progestin, rather than the addition of estrogen is responsible for higher rates of ovulation and unscheduled uterine bleeding. This study was designed to compare ovulation rates in users of a COC to users of a POP with a nearly equivalent doses of progestin.
We randomly assigned healthy women 18-45 years old who were not using hormonal birth control and who demonstrated that they were ovulatory to either a COC containing Ethinyl Estradiol 10mcg/Norethindrone acetate 1mg/iron fumarate 75mg  (Lo loestrin) or Norethindrone 1.05mg (the equivalent of three tablets of the POP Norethindrone 0.35mg) for one month.  During the 28 days of study treatment, twice weekly blood samples were collected to measure progesterone and estradiol levels, which allowed use to assess ovulation and ovarian suppression. Ovulation was defined as a single serum progesterone level of 4ng/mL or greater. Ovarian suppression was defined as all serum estradiol levels