Short Menstrual Cycles Do Not Necessarily Indicate Anovulation
Having menstrual cycles of 18-19 days for two consecutive cycles does not automatically mean you are not ovulating, but it does warrant evaluation because short cycles can indicate either ovulation with a shortened follicular phase or true anovulation. 1, 2
Understanding Short Cycles vs. Anovulation
The key distinction is that cycles of 18-19 days fall below the normal range of 21-35 days (defined as eumenorrhea), placing them in the category of polymenorrhea (shortened menstrual intervals). 3, 1 However, shortened cycles and anovulation are not synonymous:
- Ovulation can still occur in short cycles, particularly when the follicular phase is compressed but the luteal phase remains adequate. 4
- Research shows that only 3.7% of women with regular predictable cycles actually have anovulatory cycles, and many of these cases had other explanations (timing errors, isolated prolonged cycles, or age-related factors). 5
- True anovulatory cycles are most commonly associated with irregular, unpredictable bleeding patterns rather than consistently short but regular cycles. 6, 7
What Short Cycles Actually Indicate
Short menstrual cycle length (21-27 days) is significantly associated with diminished ovarian reserve, even when ovulation is occurring:
- Women with cycles of 21-27 days have AMH levels approximately 1.3 ng/mL lower than those with normal 28-31 day cycles. 4
- Antral follicle counts are approximately 5 follicles lower in women with short versus normal cycles. 4
- Fecundability in natural cycles is reduced by 19% (OR 0.81) in women with short cycles compared to normal cycles. 4
- Short cycles may represent ovarian aging with accelerated follicular recruitment, not necessarily absent ovulation. 4
Diagnostic Approach for Your Situation
To determine if you are ovulating with these 18-19 day cycles, measure mid-luteal progesterone:
- Draw progesterone 7 days before your expected next period (approximately cycle day 11-12 in your case). 5
- Progesterone ≥15 nmol/L (approximately 5 ng/mL) confirms ovulation; values <6 nmol/L suggest anovulation. 2, 8, 5
- Consider tracking basal body temperature for a biphasic pattern (temperature rise after ovulation) as additional evidence. 7
Additional workup should include:
- Pregnancy test to exclude early pregnancy or miscarriage causing irregular bleeding. 2
- TSH and prolactin levels to rule out thyroid dysfunction and hyperprolactinemia, which commonly cause cycle irregularities. 2
- LH, FSH measured on cycle days 3-6 (averaged over three measurements 20 minutes apart) to assess for PCOS (LH:FSH ratio >2) or premature ovarian insufficiency. 2
- Document your cycles for at least 6 months to determine if this pattern is consistent or transient. 2
Clinical Implications
Even if you are ovulating, consistently short cycles warrant attention:
- Persistent anovulatory cycles lead to decreased bone mineral density and increased osteoporosis risk due to hypoestrogenic states. 2
- Women with menstrual irregularities have higher rates of hypertension, metabolic syndrome, coronary heart disease, and type 2 diabetes. 2
- Fertility may be compromised even with ovulation occurring, due to reduced ovarian reserve markers and shortened fertile window. 4
Common Pitfalls to Avoid
Do not assume regular bleeding equals normal ovulation—the timing of progesterone testing is critical and must be adjusted for your shorter cycle length. 5 Do not wait for three missed cycles (the definition of secondary amenorrhea) before seeking evaluation when cycles are consistently abnormal. 3, 1 Do not rely on cycle-tracking apps alone—these have only 21% accuracy for predicting ovulation and cannot account for individual variation. 9