Due Date vs. Conception Date: How They Differ and How to Calculate Both

Your due date is calculated from the first day of your last menstrual period (LMP) and spans 280 days, or 40 weeks. Your conception date is about 14 days after LMP, when ovulation and fertilization usually occur. Both describe the same pregnancy from different starting points. OBs use LMP because it is the more reliable reference point for most people.

The two reference points

Pregnancy has two natural markers. The LMP is the first day of the last menstrual period before conception. The conception date is the day sperm fertilized the egg, which happens around ovulation. In a regular 28-day cycle, ovulation and conception fall around day 14 of the cycle, so LMP is roughly 14 days earlier than conception. A pregnancy measured from LMP is 40 weeks long, while the same pregnancy measured from conception is closer to 38 weeks. Both numbers describe the same 266 days of actual gestation.

How the 280-day rule works

Naegele's rule, the standard formula used by obstetric guidelines, takes LMP, adds one year, subtracts three months, and adds seven days. That produces the 40-week due date. Mathematically it is the same as adding 280 days to LMP. You can see this applied in our Due Date Calculator, which supports both LMP input and conception-date input and shows you the trimester and remaining weeks.

Why doctors use LMP, not conception

Most people can remember the first day of their last period with reasonable accuracy. Far fewer know their exact day of ovulation without specific tracking. ACOG Committee Opinion 700 and NICE antenatal guidelines both default to LMP because it is the most universally available and reproducible reference. Conception dating is used when LMP is unknown or unreliable, for example in irregular cycles, after hormonal birth control, or when the person only started tracking mid-cycle.

When conception dating is better

Conception dating wins in three scenarios. First, IVF transfers, where the exact embryo transfer date is known and the due date is calculated precisely from that event. Second, known ovulation from LH testing, basal body temperature charting, or cycle monitoring in a fertility clinic. Third, recent hormonal contraception, where the last "period" was a withdrawal bleed and does not reflect a natural cycle. In any of these cases, your clinician will typically calculate the due date from conception directly. Our Conception Calculator works backwards from a known or suspected conception date and returns the matching LMP and due-date estimates.

When an ultrasound overrides everything

A first-trimester ultrasound that measures the crown-rump length (CRL) is more accurate than any LMP-based estimate. ACOG's guidance is clear: if the ultrasound-based due date disagrees with the LMP-based date by more than 5 to 7 days in the first trimester, the ultrasound date takes over. By the second trimester the tolerance for disagreement widens because fetal size varies more between babies. After 14 weeks the LMP estimate is generally retained unless there is no reliable LMP at all.

How accurate is any of this

About 4 percent of babies are born on their exact due date. Roughly 70 percent arrive within 10 days of the estimate, and 90 percent within 2 weeks. Full term is 37 to 42 weeks, and the statistical mode is 40 weeks 5 days for first pregnancies. Treat the due date as the middle of a bell curve, not a promise, and do not be surprised if your plans shift by a week or two in either direction.

FAQ

Why is due date calculated from the last period, not conception?

Most people can reliably identify LMP. Fewer know exactly when they ovulated. LMP is the default in ACOG and NICE guidelines because it is the most universal reference.

What if my cycle is not 28 days?

Naegele's rule assumes a 28-day cycle. Longer or shorter cycles shift ovulation away from day 14, and a first-trimester ultrasound typically corrects the date more accurately than a manual adjustment.

How accurate is the due date?

About 4 percent of babies arrive on their due date. Around 70 percent arrive within 10 days and 90 percent within 14 days. Full term is 37 to 42 weeks.

Can an ultrasound override LMP dating?

Yes. In the first trimester, a CRL ultrasound that disagrees with LMP by more than 5 to 7 days takes over as the due-date source.

Should I trust a conception-based estimate?

It is only as good as the conception date. IVF or tracked ovulation gives a very accurate estimate. Guessing at conception is usually less accurate than using LMP.

Bottom line

Due date and conception date describe the same pregnancy measured from different reference points, separated by roughly 14 days. OB guidelines default to LMP because it is the more reliable universal marker. If you have a known conception date from IVF or cycle tracking, use that. A first-trimester ultrasound is the tiebreaker. For a quick calculation, try our Due Date Calculator or the Conception Calculator, which both run in your browser with no data sent to us.