Estimate conception window, due date (EDD), current gestational age, and ovulation timing based on LMP, due date, or known conception date. Custom cycle length for precision. Based on ACOG and WHO guidelines.
The conception date marks the moment of fertilization when sperm meets egg, typically occurring within 12–24 hours after ovulation. Since exact conception is often unknown, clinicians rely on the last menstrual period (LMP) or early ultrasound to estimate due date and gestational age. Our calculator uses the widely accepted Naegele's rule (LMP + 280 days) and incorporates cycle-length adjustments for more accurate ovulation/conception windows.
? Interpreting results with irregular cycles: If your cycle length varies by more than 7 days between months, the estimated ovulation day may be off by ±3–5 days. For best accuracy, use the average cycle length of your last 6 cycles. For conditions such as PCOS or breastfeeding‑related amenorrhea, early ultrasound dating is significantly more reliable than calendar methods. This tool assumes a luteal phase of 14 days (standard clinical model).
Standard formula: EDD = LMP + 280 days (40 weeks). Conception typically occurs ~14 days before next expected period → Ovulation day = LMP + (cycle length - 14) days.
Women with longer cycles (e.g., 35 days) ovulate later than day 14; those with shorter cycles ovulate earlier. By adjusting ovulation day, we provide a personalized conception window. Research from the American College of Obstetricians and Gynecologists (ACOG) confirms that cycle variability directly impacts fertile window timing. Our tool respects these biological nuances.
Our algorithm combines Naegele's rule with reverse calculations for due-date mode and conception‑date mode. For due‑date based estimation, LMP = EDD - 280 days. For known conception, EDD = conception + 266 days (typical gestation from fertilization). Each method then applies the custom cycle length to refine the ovulation/conception window. All dates are validated for consistency; if LMP is after today’s date or due date is too early, the tool displays a clear warning.
⚠️ Limitations vs. ultrasound: The calendar method assumes a perfect 280‑day gestation and a 14‑day luteal phase, but individual pregnancies can vary by up to 2 weeks. First‑trimester ultrasound (crown‑rump length) has an error margin of ±5–7 days, while second‑trimester ultrasound ±10–14 days. Therefore, if you have undergone an ultrasound, your healthcare provider's adjusted due date should take precedence over this calculator.
This tool is ideal for confirming pregnancy timing, planning prenatal appointments, understanding fetal development stages, and educating about fertility awareness.
The fertile window spans approximately 6 days: the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the female reproductive tract, while the egg remains viable for ~24 hours after ovulation. Therefore, the conception window typically covers ovulation day ± 2 days. Our calculator displays a personalized range based on your cycle length and chosen mode, helping you understand the likely date of fertilization.
A user with LMP = January 1, 2026, and cycle length = 32 days will have estimated ovulation on January 19 (LMP + 18 days). Due date = October 8, 2026. Conception window: January 17–21. This refined estimate accounts for later ovulation, preventing misjudgment of gestational age.
Our calculator shows current trimester based on today’s date relative to LMP/EDD. This helps you track prenatal milestones, schedule screenings, and understand fetal development stages.