Calculate your likely conception date from your due date or LMP. Shows the estimated conception window (fertile days), ovulation date, and how conception date relates to gestational age. Works both forward and backward.
Forward: enter your LMP to calculate your likely conception date and fertile window. Backward: enter your known due date to calculate when conception likely occurred. Both methods show the 5–7 day conception window around ovulation.
Enter either your LMP (first day of last period) or your due date. For LMP-based calculation, enter your average cycle length to pinpoint ovulation day. Ovulation typically occurs on cycle day 14 for a 28-day cycle, but shifts with cycle length.
The estimated ovulation date and a 5-day fertile window are shown. The window includes 5 days before ovulation and 1 day after — the period when intercourse can result in pregnancy. The exact conception date is an estimate; sperm survive 3–5 days in the reproductive tract.
Working backward: conception typically occurs 266 days (38 weeks) before the due date. If the EDD is 1 January, then conception was approximately 266 days earlier — around 10 April of the previous year. This gives the estimated conception date. Ovulation (which immediately precedes conception) occurred around the same date, and the fertile window was 5 days before through 1 day after ovulation.
The conception date is an estimate with an uncertainty of ±5–7 days for most pregnancies. Sperm can survive in the reproductive tract for 3–5 days. If intercourse occurred 4 days before ovulation and sperm survived until ovulation, conception happened on ovulation day — not on the day of intercourse. The fertile window accounts for this by showing a range of days when conception was possible.
The fertile window is the days of a menstrual cycle when pregnancy is possible. Sperm survive 3–5 days; an egg survives 12–24 hours after ovulation. Therefore the fertile window is approximately 6 days: 5 days before ovulation and the day of ovulation itself. The most fertile days are the 2 days immediately before ovulation. For a 28-day cycle, the fertile window is approximately days 10–15, with peak fertility on days 12–14.
This calculator estimates a conception window — typically 5–7 days — not a single day. For legal paternity determination, DNA testing is the only reliable and legally recognised method. The conception window estimate cannot definitively establish who the father is when multiple partners were involved within the fertile window. Always consult a legal professional and healthcare provider for paternity questions.
An egg (ovum) is viable for only 12–24 hours after ovulation. Sperm must already be present in the fallopian tube to fertilise it. Sperm deposited in the reproductive tract before ovulation can survive 3–5 days awaiting the egg. This is why intercourse in the days leading up to ovulation is most likely to result in conception. Sperm deposited more than 5 days before ovulation are typically too degraded to fertilise.
The due date calculator goes forward from LMP to estimate the EDD. The conception calculator focuses on pinpointing when conception occurred — either by going backward from an EDD or forward from an LMP, with specific attention to the fertile window and ovulation timing. Use the due date calculator if you want your EDD; use the conception calculator if you want to understand when and how conception likely occurred.
