Pregnancy Timeline
Estimate your likely conception date and fertile window.
Estimated Conception Date
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Enter your LMP date and cycle length above.
These are estimates based on standard pregnancy timelines. Actual conception date may vary by 1–2 days. Consult your healthcare provider for confirmation via ultrasound.
Conception typically occurs around ovulation, which happens roughly 14 days after the first day of your last menstrual period (LMP), though this varies with cycle length. This calculator estimates your conception date and shows your fertile window (the 5 days before and 1 day after conception, when pregnancy is most likely). You can calculate either from your LMP date or work backwards from an expected due date.
If you enter your LMP date and cycle length, the calculator estimates ovulation (14 days + cycle adjustment), derives conception from that, and calculates your expected due date (280 days from LMP). If you enter a due date, it works backwards: due date minus 266 days gives conception; conception plus 14 days gives LMP. The tool also shows your current gestational age (weeks and days since LMP).
Conception ≈ LMP + (14 + (cycle length − 28)) days; Due Date ≈ LMP + 280 days; Fertile Window ≈ Conception −5 to +1 daysLMP is the first day of your last menstrual cycle. Ovulation (and conception) typically occur around day 14, adjusted by how many days your cycle differs from 28. Standard pregnancy duration is 280 days from LMP (266 days from conception). The fertile window spans 5 days before ovulation through 1 day after, because sperm survive up to 5 days and the egg survives about 1 day.
| Input | Result | Notes |
|---|---|---|
| LMP: 1 January 2024, Cycle: 28 days | Conception: ~15 January 2024; Due Date: ~10 October 2024 | Standard 28-day cycle; ovulation and conception on day 14. |
| LMP: 15 February 2024, Cycle: 32 days | Conception: ~18 February 2024; Due Date: ~24 November 2024 | 32-day cycle (4 days longer); ovulation shifts to day 18, so conception is 4 days later. |
| Due Date: 20 December 2024 | Conception: ~15 March 2024; LMP: ~2 March 2024 | Working backwards from due date; no cycle adjustment needed. |
The fertile window is the 5–6 days when pregnancy is most likely. It includes the 5 days before ovulation and the 1 day after ovulation (when the egg is still alive). Sperm can survive up to 5 days in the reproductive tract.
No. Ovulation is when the egg is released (roughly day 14 of a 28-day cycle). Conception occurs when sperm fertilises the egg, usually within 12–24 hours of ovulation. If intercourse happens before ovulation, sperm wait, so conception may occur a few days after intercourse.
Use your average cycle length (count days from the first day of one period to the first day of the next, over 2–3 months). If your cycle is very irregular (e.g., 21–35 days), use the midpoint or consult a healthcare provider; an ultrasound is more reliable.
Conception estimates are typically accurate to within 1–2 days when you know your exact LMP and cycle length. Ultrasound in the first 13 weeks is more precise (±3–5 days); later ultrasounds are less accurate (±2–3 weeks in the second trimester).
This calculator supports cycles from 20–35 days. If your cycle is longer, consult your healthcare provider; very long cycles may indicate hormonal or health conditions needing evaluation.
An ultrasound is your best option; it can estimate gestational age and due date even if you don't know LMP. Ultrasound is most accurate in the first trimester, less so later. Consult your healthcare provider.