Depot medroxyprogesterone acetate (DMPA) is a highly effective contraceptive when injected during the first seven days of the menstrual cycle. However, recent studies have suggested that DMPA given on days 9 through 11 of the cycle may not suppress ovulation. Because of the difficulty of timing DMPA injections with a patient's cycle, the current recommendation is to provide the injection any time during the menstrual cycle as long as the woman is not pregnant. Petta and colleagues studied the effect of DMPA injections on ovarian function in women who were at days 8 through 13 of the menstrual cycle.

Thirty healthy women between 18 and 40 years of age were enrolled in the study. Exclusion criteria included use of hormonal contraception in the past four months and recent pregnancy or breast feeding. All of the patients were seen and evaluated once during days 8 through 13 of the menstrual cycle. Ovarian function was evaluated by transvaginal ultrasound examination and serum estrogen levels. The women were give 150 mg of DMPA and underwent evaluation for ovulation at days 1, 3 and 7 by repeat ultrasound examinations and serum estrogen and luteinizing hormone levels. Serum progesterone levels were measured at day 7 after the injection and, if the level was more than 2.5 ng per mL (7.9 nmol per L) and follicular rupture was observed by ultrasound, the patients were assumed to have ovulated during the study.

None of the women who were on days 8 or 9 of their cycle ovulated after DMPA injections. One of five women on day 10 of the cycle ovulated, as well as four of 10 women who were on day 11 or 12 of the cycle and all but one of the women on day 13 of the cycle. In all, 30 percent (nine of 30 women) of the women ovulated during the study after an injection of DMPA. All women who ovulated did so within three days of the injection, suggesting that DMPA may trigger ovulation if given close to mid-cycle.

Ovulation suppression with DMPA is related to the timing of administration and women's cycles. During the first seven days after the start of the cycle, it provides good suppression of ovulation. However, if DMPA is given on days 8 through 13 of the menstrual cycle, the injections may not reliably suppress ovulation. The authors conclude that women who receive DMPA injections during days 8 through 13 of the menstrual cycle should use a back-up method of contraception for seven days to reduce the risk of unintended pregnancy.