DMPA Contraceptive Failure Analysis
DMPA failed because the injection was administered outside the optimal initiation window (beyond 7 days after the last menstrual period started) without adequate backup contraception, allowing ovulation and conception to occur before the contraceptive became fully effective.
Mechanism of Failure
The most likely scenario involves timing of administration relative to the menstrual cycle and ovulation:
DMPA requires 7 days to achieve full contraceptive effectiveness when started outside the first 5-7 days of the menstrual cycle, during which time backup contraception (condoms or abstinence) is mandatory 1.
MPA becomes detectable in serum within 30 minutes of injection, but ovulation suppression is not immediate—cervical mucus changes occur within 24 hours in only 90% of women, and full contraceptive protection requires consistent serum levels 1, 2.
If the injection was given more than 7 days after LMP and unprotected intercourse occurred in the 5 days before or 7 days after the injection, pregnancy could result from either pre-existing sperm (viable up to 5 days) or ovulation occurring before adequate MPA levels were established 1, 3.
Critical Timing Failures
Three specific scenarios lead to DMPA failure:
Late cycle initiation without backup: If DMPA was started on cycle day 8 or later without 7 consecutive days of backup contraception, ovulation may have already occurred or been imminent 1, 3.
Failure to use emergency contraception: If unprotected intercourse occurred in the 5 days before receiving the injection (especially during the first week of the cycle), emergency contraception should have been considered but was not provided 1.
Premature discontinuation of prior contraception: If the patient discontinued a previous contraceptive method prematurely when receiving DMPA outside the optimal window, a contraceptive gap occurred 1.
Pharmacokinetic Considerations
Serum MPA levels plateau at approximately 1.0 ng/mL for about 3 months after injection, but initial levels sufficient to inhibit ovulation take time to establish 2.
Ovulation inhibition requires sustained MPA levels above 0.1 ng/mL; when started mid-cycle, there may be insufficient time to prevent an already-developing follicle from ovulating 2.
The LH surge can occur before adequate contraceptive levels are achieved if injection timing coincides with the late follicular phase 2.
Common Pitfalls in DMPA Initiation
Provider errors that lead to failure:
Not verifying the patient had reasonable certainty of not being pregnant before administration (though pregnancy testing is not mandatory, clinical assessment is required) 1, 4.
Failing to counsel about the mandatory 7-day backup contraception requirement when starting outside days 1-7 of menses 1.
Not assessing for recent unprotected intercourse that would necessitate emergency contraception before or concurrent with DMPA initiation 1.
Allowing patients to discontinue their current contraceptive method before DMPA becomes fully effective 1.
Optimal Initiation Protocol to Prevent Failure
To avoid contraceptive failure, DMPA should be initiated:
During days 1-7 of a normal menstrual cycle (no backup needed) 1, 4.
At any time if reasonably certain the patient is not pregnant, with mandatory backup contraception for 7 consecutive days if more than 7 days since menses started 1, 4.
Immediately post-abortion (surgical or medication) with no backup needed if given at the time of the procedure 1, 4.
Immediately postpartum, with backup requirements based on timing and breastfeeding status 1, 4.
Verification of Pregnancy Status
While not mandatory, establishing reasonable certainty of non-pregnancy is essential 1, 5:
- Recent normal menses with no unprotected intercourse since
- No signs or symptoms of pregnancy
- Correct and consistent use of another contraceptive method
- Within 7 days of normal menses
- Within 4 weeks postpartum (non-lactating)
- Within 7 days post-abortion
If uncertainty exists, DMPA can still be started with follow-up pregnancy testing in 2-4 weeks, as the benefits of preventing pregnancy typically outweigh theoretical risks 5.