Conception Date Calculator

Work backwards from your due date to estimate when conception likely occurred, or forward from your last period date. Shows gestational age, trimester, and conception window.

The conception date calculator works in two directions: from a known due date backward to estimate the likely conception window, or from the last menstrual period (LMP) forward to estimate the due date and conception date. Clinical due dates (EDC — estimated date of confinement) are calculated using Naegele's rule, which adds 280 days (40 weeks) to the LMP. Conception itself typically occurs around day 14 of a 28-day cycle — 2 weeks after LMP. BMI Calculator and Daily Calories Calculator are closely related tools for tracking fertility and reproductive health.

The 2–5 day conception window shown in results reflects sperm viability (up to 5 days) and egg viability (12–24 hours after ovulation). Because ovulation is not directly observable without LH testing or BBT tracking, the "conception window" is the scientific best estimate rather than a precise date. Ultrasound dating in early pregnancy (before 14 weeks) is more accurate than LMP-based calculation if the two methods give different dates.

Choose your mode: "I know my due date" works backwards to estimate conception; "I know my last period date" works forwards to estimate the due date.
If using due date mode, enter the due date given by your midwife or scan — the calculator will estimate your LMP, ovulation date, and conception window.
If using LMP mode, enter the first day of your last period and your cycle length (default 28 days).
Read the conception window (typically a 5-day range around estimated ovulation), estimated due date, and current gestational age and trimester if applicable.
Note: if your due date was set by an early ultrasound scan, use the scan date rather than LMP calculation — scan dating is more accurate in the first trimester.

The conception date and due date formula explained

Due date (Naegele's rule) = LMP + 280 days (or equivalently: LMP + 9 months + 7 days). Estimated conception = LMP + (cycle length − 14) days ± 2–3 days. Reverse: LMP = Due date − 280 days; Conception window = Due date − 266 days (±3 days).

The 280-day (40-week) gestational period is measured from LMP, not from conception. Conception occurs approximately 14 days after LMP (for a 28-day cycle), so actual fertilisation-to-birth duration is approximately 266 days (38 weeks). This distinction matters when working backwards from a due date — the calculator subtracts 266 days from the due date to find the estimated conception date, and 280 days to find LMP.

Worked example: due date 15 January. LMP estimate = 15 Jan − 280 days = 9 April (prior year). Estimated ovulation = 9 April + 14 days = 23 April. Conception window = 20 April – 25 April. Current gestational age (from today's date) = number of days from estimated LMP ÷ 7 = weeks + days.

Understanding your conception date result

Trimesters, gestational age, and scan dating

Pregnancy is divided into three trimesters: first trimester (weeks 1–12), second trimester (weeks 13–27), and third trimester (weeks 28–40). Gestational age is always counted from LMP, not from conception. A "full term" pregnancy is defined as 39–40 weeks; 37–38 weeks is "early term"; 41–42 weeks is "late term." Delivery before 37 weeks is preterm. The due date is an estimate — only about 5% of births occur exactly on the calculated date; 80% occur within 2 weeks either side.

If your clinically assigned due date differs from the LMP-calculated date by more than 7 days (first trimester scan) or 14 days (second trimester scan), the scan date takes precedence in clinical management. Use Protein Intake Calculator to track the fertile window for planning future pregnancies.

Health tips and best practices

If you had an early ultrasound (before 14 weeks), use the scan-assigned due date rather than the LMP calculation — crown-rump length measurement is more accurate than LMP dating.
Keep a note of your LMP date even after a positive pregnancy test — you will be asked for it at every antenatal appointment.
The conception window is based on a 28-day cycle default — if your cycle is shorter or longer, adjust accordingly to improve accuracy.
Paternity window questions require a legal or clinical expert — this calculator estimates conception windows for general informational purposes only and cannot establish paternity.
Start folic acid (400 mcg/day) as soon as you plan to conceive, not just after a positive test — neural tube closure occurs in the first 4 weeks, often before a test is positive.
Morning sickness typically begins at 6 weeks gestational age (4 weeks after conception) and peaks around 9–10 weeks — use the gestational age shown to anticipate timing.

Only about 4–5% of babies are born on their calculated due date; 80% are born within 2 weeks of it (NICE, 2021).
Spontaneous labour at 39–40 weeks has the lowest associated risk for both mother and baby; induction before 39 weeks without medical indication increases complication risk.
The average gestational length from ovulation to birth is 268 days (just over 38 weeks), very close to the 266-day estimate used here (BMJ, 2013).
Approximately 11% of pregnancies globally end in preterm birth (<37 weeks); in the UK the rate is approximately 7.5% (MBRRACE-UK, 2022).

Common mistakes to avoid

Confusing the due date with the conception date — the due date is 40 weeks from LMP, but conception is approximately 2 weeks after LMP (38 weeks before birth).
Using this calculator to determine paternity — the conception window is an estimate with a margin of error of several days and is not legally or medically definitive.
Ignoring a scan-assigned due date in favour of the LMP calculation — ultrasound is more accurate, and using two different dates creates clinical confusion.
Assuming the due date is a deadline — it is a statistical midpoint; only 5% of births happen on the exact date.

Due date calculation uses Naegele's rule as recommended by NICE (National Institute for Health and Care Excellence) and ACOG (American College of Obstetricians and Gynecologists). Conception window estimates are based on standard ovulation timing and published sperm/egg viability data. This tool provides general educational information only and cannot confirm pregnancy, establish paternity, or substitute for clinical obstetric care. Gestational age in clinical management is determined by your midwife or obstetrician using ultrasound and LMP data together. Always discuss pregnancy dating with your healthcare team.