Period & Cycle Tracker

The menstrual cycle is a hormonal cycle regulated by the hypothalamus, pituitary gland, and ovaries. It's divided into four phases: menstruation, the follicular phase, ovulation, and the luteal phase. Understanding these phases helps explain why predictions are most reliable in the middle of the cycle and why stress, illness, or travel can shift your timing.

The four phases

• Menstruation (Days 1–5): The uterine lining sheds as estrogen and progesterone levels drop. Day 1 is always the first day of true flow — not spotting. Duration is typically 3–7 days.
• Follicular phase (Days 1–13): Follicle-stimulating hormone (FSH) triggers follicle development in the ovaries. Estrogen rises, the uterine lining thickens. One dominant follicle emerges and produces increasing estrogen, which eventually triggers the LH surge.
• Ovulation (around Day 14 in a 28-day cycle): A surge in luteinizing hormone (LH) — detectable with OPK strips 24–48 hours before ovulation — causes the dominant follicle to rupture and release the egg. The egg survives 12–24 hours. This is the only window for fertilization.
• Luteal phase (Days 15–28): The ruptured follicle becomes the corpus luteum, producing progesterone. This phase is relatively constant at 12–16 days across most people — cycle length variability comes from the follicular phase, not the luteal phase. If fertilization doesn't occur, progesterone drops and menstruation begins.

Why cycles vary

Cycle length variation almost always comes from the follicular phase. Stress, illness, significant weight change, thyroid dysfunction, PCOS, and perimenopause all affect follicle development and can delay or suppress ovulation. The luteal phase is physiologically fixed by progesterone production from the corpus luteum. This is why "late ovulation" causes a late period — it's not that your cycle extended after ovulation, it's that ovulation itself was delayed.

ACOG considers cycles of 21–35 days normal in reproductive-age individuals. Cycles consistently outside that range — or cycles varying by more than 7–9 days from cycle to cycle — warrant evaluation for PCOS, thyroid disease, hyperprolactinemia, or premature ovarian insufficiency.

Using this tracker for TTC

Calendar-based prediction has real limits for timing intercourse when trying to conceive (TTC). The fertile window is approximately 6 days: the 5 days before ovulation and the day of ovulation itself. Calendar prediction works best when cycles are regular. For more precise ovulation timing, combine calendar tracking with OPKs (which detect the LH surge 24–48 hours before ovulation) and/or basal body temperature charting (which confirms ovulation occurred via a temperature rise of 0.2–0.5°F). Per Wilcox et al. (1995), the two days before ovulation and the day of ovulation itself carry the highest per-cycle conception probability.