Lo Loestrin Effect on Progesterone
Lo Loestrin (ethinyl estradiol 20 µg/norethindrone acetate 1 mg) suppresses endogenous progesterone production through its mechanism as a combined oral contraceptive, and supplemental progesterone is neither required nor recommended for contraceptive efficacy or cycle control. 1
Mechanism of Progesterone Suppression
Combined oral contraceptives containing ethinyl estradiol and norethindrone acetate work by inhibiting gonadotropin-releasing hormone, follicle-stimulating hormone, and luteinizing hormone, which prevents ovulation and consequently suppresses endogenous progesterone production from the corpus luteum. 1
The progestin component (norethindrone acetate) is a first-generation synthetic progesterone analogue derived from testosterone that provides the progestational effects needed for contraception. 1
When combined with ethinyl estradiol, the formulation achieves net antiandrogenic properties despite norethindrone acetate's androgenic potential when used alone. 1
Seven-Day Rule for Ovarian Suppression
Seven consecutive days of combined hormonal contraceptive use is necessary to reliably suppress ovulation and maintain contraceptive efficacy. 1
Studies examining deliberate pill omission show that missing pills mid-cycle generally results in low risk of ovulation, though some follicular development may occur. 1
Research specifically examining 20 µg ethinyl estradiol formulations demonstrated continued ovarian steroidogenesis when pills were missed, but no breakthrough ovulation occurred with one or two missed pills mid-cycle. 2
Lower-dose ethinyl estradiol formulations (20 µg) show more follicular activity when pills are missed compared to 30 µg formulations, making consistent daily use particularly important. 1
No Role for Supplemental Progesterone
Supplemental progesterone should not be prescribed routinely with combined oral contraceptives, as there is no evidence of benefit and the synthetic progestin in the formulation provides adequate progestational effects. 3
The norethindrone acetate component provides sufficient progestational activity to maintain endometrial stability and contraceptive efficacy. 1
Progesterone supplementation is only indicated in pregnancy when there is clear evidence of luteal phase insufficiency or specific obstetric indications—not for contraceptive use. 3
Clinical Considerations for Lo Loestrin Specifically
Lo Loestrin's 24-day active hormone regimen (compared to traditional 21-day regimens) was specifically designed to improve cycle control while maintaining the low 20 µg ethinyl estradiol dose. 4
The 24-day formulation of norethindrone acetate 1 mg/ethinyl estradiol 20 µg demonstrated significantly fewer intracyclic bleeding days (0.95 vs 1.63 days) and fewer total bleeding/spotting days compared to the 21-day regimen of the same hormones. 4
Cycle control with 20 µg ethinyl estradiol formulations varies by progestin type; norethindrone acetate formulations historically show higher rates of breakthrough bleeding and amenorrhea (10% amenorrhea rate) compared to levonorgestrel-containing 20 µg formulations. 5
Common Pitfall to Avoid
Do not prescribe progestin-only pills, injections, implants, or intrauterine devices as "progesterone supplementation" for patients on combined oral contraceptives—these may worsen acne and provide no contraceptive benefit when combined with COCs. 1
- If a patient requires additional hormonal support for a specific medical indication, this represents a separate clinical decision that should be made independently of their contraceptive regimen.