Conception Date from LMP — Last Menstrual Period to Conception

The LMP to Conception Formula

Ovulation typically occurs 14 days before the start of the next menstrual period. Therefore, for any cycle length, the ovulation day equals cycle length minus 14. The conception date formula is: LMP date plus (cycle length minus 14). For a 28-day cycle, ovulation occurs on day 14, so conception is estimated at LMP plus 14 days. For a 30-day cycle, ovulation is on day 16, making conception LMP plus 16 days. For a 35-day cycle, ovulation is on day 21, so conception is LMP plus 21 days. This formula relies on the fact that the luteal phase — the period from ovulation to the start of the next period — is relatively constant at about 14 days for most individuals.

28-Day Cycle Example

For someone with a regular 28-day cycle and an LMP of February 1, ovulation is estimated around day 14, which is February 15. Sperm deposited during the fertile window (days 9 to 14) can survive in the reproductive tract and meet the egg upon its release. The egg survives 12 to 24 hours, so fertilization most likely occurs on February 15 or the morning of February 16. The estimated conception date is February 15. Pregnancy dating conventions then consider this woman to be about 2 weeks pregnant at the moment of conception, with a full-term due date 280 days from February 1, or approximately November 2.

35-Day Cycle Example

For someone with a regular 35-day cycle and an LMP of February 1, ovulation is estimated around day 21, which is February 22. The fertile window opens about 5 days before ovulation, starting around day 16 (February 17), and closes about a day after ovulation. Conception is most likely on February 22. This woman would be considered about 3 weeks pregnant at the moment of conception under the LMP dating convention. Her due date would be calculated as 280 days from February 1, or approximately November 9 — the same as a 28-day cycle from the same LMP, unless the calculation is adjusted for the longer cycle length. If the conception date is used directly, the due date is 266 days from February 22, which is November 15.

Ovulation Signs to Confirm the Date

The LMP formula provides an estimate; ovulation signs confirm the actual date. A sustained basal body temperature rise of 0.3 to 0.5 degrees Celsius indicates that ovulation has occurred, as progesterone produced by the corpus luteum raises body temperature. Fertile-type cervical mucus — clear, stretchy, and egg-white in consistency — appears 2 to 3 days before ovulation and persists through the day of ovulation. An ovulation predictor kit (OPK) detects the luteinizing hormone (LH) surge, which precedes ovulation by 24 to 36 hours. Mild one-sided lower abdominal pain (mittelschmerz) can occur at ovulation. Mild breast tenderness, increased libido, and light spotting can also occur around ovulation but are less specific. Using two or more of these signs together gives the most accurate ovulation and conception date.

When to See a Doctor About Conception

If you have been trying to conceive for 12 months (or 6 months if age 35 or older) without success, the American Society for Reproductive Medicine (ASRM) recommends a fertility evaluation. Irregular or absent menstrual periods for 3 or more months also warrant earlier evaluation. A healthcare provider can assess ovulation function through hormone blood tests (follicle-stimulating hormone, luteinizing hormone, progesterone on day 21 of a 28-day cycle), pelvic ultrasound to evaluate ovarian and uterine anatomy, and thyroid function testing. Early evaluation is also recommended for known conditions such as polycystic ovary syndrome (PCOS), endometriosis, or a history of pelvic inflammatory disease. A reproductive endocrinologist can provide targeted testing and treatment options.