Conception Date Calculator

Pregnancy dating does not start at the moment of conception — it starts two weeks earlier, from the first day of the last menstrual period. This counterintuitive convention exists because the LMP is a concrete, observable date, while conception cannot be directly observed. By the time a pregnancy is confirmed, the LMP is already the standard reference point in obstetrics.

The core calculation uses Naegele rule, established in the 19th century and still the backbone of modern obstetric dating. Add 280 days (40 weeks) to the LMP to get the estimated due date, or run it in reverse: subtract 280 days from the due date to get the estimated LMP. Conception is then estimated by adding the ovulation offset — 14 days after the first day of the LMP in a 28-day cycle — to the LMP. This places conception about 14 days before the next expected period, which is when ovulation and fertilization most likely occurred.

The fertile window accounts for biological reality: sperm can survive in the fallopian tubes for up to 5 days, and an egg remains viable for 12 to 24 hours after ovulation. This means intercourse that occurred several days before ovulation can still result in conception. The window shown runs from 5 days before the estimated ovulation date to 1 day after it, 7 days in all. ACOG puts it this way: "You can become pregnant if you have sex anywhere from 5 days before ovulation until 1 day after ovulation."

This tool is appropriate when you want a quick, reasonable estimate of conception timing based on a known due date or LMP. Common uses include understanding your own pregnancy timeline, confirming that a stated conception date is consistent with your gestational age, or checking whether a specific date falls within the fertile window.

It is also useful for people who conceived through timed intercourse or ovulation tracking and want to cross-check their clinical dating against their own records. If your tracked ovulation date and this calculator agree within a day or two, you can be reasonably confident in the timeline.

This tool is not appropriate as a substitute for clinical dating. If you have an irregular cycle, a history of late ovulation, or an ultrasound date that conflicts with your LMP, the estimated conception date here may be off by more than the standard window. In those cases, the ultrasound measurement — specifically crown-rump length in the first trimester — is more accurate than any date-based calculation and should take precedence.

The most common mistake is assuming a 28-day cycle when the actual cycle is significantly different. A woman with a 35-day cycle who uses a standard calculator will get a conception date that is 7 days too early. This matters in situations where the timeline is important — paternity questions, in vitro fertilization records, or reconciling an ultrasound date with a known event. Always enter your actual average cycle length.

A second frequent error is confusing ultrasound gestational age with calendar age. An ultrasound performed at 8 weeks gestation dates the pregnancy from the LMP, not from conception. The baby is biologically about 6 weeks old at that point. When a clinician says 8 weeks, they mean 8 weeks from LMP — which means conception was approximately 6 weeks ago. Using the gestational age directly as the age since conception will throw off the calculation by about 2 weeks.

The third mistake is treating the output as a precise date rather than a probability window. Even with a known LMP and cycle length, conception can vary by several days. When someone uses this calculator to answer a specific paternity question or confirm an exact event date, they should work with the full 7-day fertile window, not just the single date. A single date feels certain — a window is honest.

The calculation has two paths depending on what date you start from.

Starting from the due date: LMP = Due Date minus 280 days. Conception Date = LMP plus ovulation offset. Ovulation offset = cycle length minus 14 days, because ovulation falls about 14 days before the next period (ACOG; the NHS gives 10 to 16 days), which starts one full cycle length after the LMP. For a 28-day cycle: offset = 14. For a 32-day cycle: offset = 18. For a 24-day cycle: offset = 10.

Starting from the LMP: Due Date = LMP plus 280 days. Conception Date = LMP plus ovulation offset. Both paths converge on the same conception date — the only variable is which anchor date you provide.

Fertile window boundaries (ACOG): window start = Conception Date minus 5 calendar days, because sperm can survive up to 5 days. Window end = Conception Date plus 1 calendar day, because the egg lives 12 to 24 hours. The window leans earlier than the ovulation date because sperm survival reaches further back than egg viability reaches forward.

Naegele rule was derived from a uniform 28-day cycle assumption and overstates certainty for anyone outside that norm. The formula treats conception as a point event at ovulation, but fertilization can occur up to 24 hours after ovulation and implantation follows 6 to 12 days later. First-trimester ultrasound dating — using crown-rump length — has a margin of error of plus or minus 5 to 7 days, which is why clinical due dates rarely change after the first ultrasound but may differ from LMP-based calculations. For cycles significantly longer than 28 days, the LMP-based due date will be systematically later than the true due date, because the formula assumes ovulation 14 days after the period starts regardless.