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. LMP is commonly used because exact conception timing is often unknown; a first-trimester ultrasound is the more accurate way to confirm or revise the due date.

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 LMP is the common starting point

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. LMP is a practical and reproducible reference when exact conception timing is unknown, which is most of the time. ACOG Committee Opinion 700 still identifies first-trimester ultrasound as the most accurate method to establish or confirm gestational age, and recommends that the final EDD be based on the best obstetric estimate rather than LMP alone. NICE NG201 recommends offering a dating ultrasound between 11 weeks 2 days and 14 weeks 1 day. 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. Per ACOG Committee Opinion 700, the ultrasound-based date takes over when the discrepancy exceeds defined thresholds: more than 5 days before 9 weeks, more than 7 days at 9 to 13 weeks, and larger tolerances at later gestational ages (7 days at 14 to 15 weeks, 10 days at 16 to 21 weeks, 14 days at 22 to 27 weeks). After the first trimester, ultrasound can still change the EDD when the discrepancy is large enough under those thresholds.

How accurate is any of this

Few babies are born on their exact due date. Pregnancy normally lasts 37 to 42 weeks from the first day of the last period (NHS). Jukic et al. (2013) found healthy singleton gestational length varies by about 37 days between pregnancies, with a median of 268 days from ovulation, and reported no association with parity. Treat the due date as the middle of a wide window rather than a promise.

FAQ

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

Most people can reliably identify LMP; fewer know their exact day of ovulation. LMP is a practical reference when exact conception timing is unknown. Per ACOG Committee Opinion 700, the final EDD should be based on the best obstetric estimate, and a first-trimester ultrasound is the most accurate method to confirm gestational age.

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?

Few babies arrive exactly on their due date. Pregnancy normally lasts 37 to 42 weeks from the first day of the last period (NHS). Healthy singleton gestational length varies by about 37 days per Jukic et al. (2013).

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. LMP is a practical starting point when exact conception timing is unknown. If you have a known conception date from IVF or cycle tracking, use that. A first-trimester ultrasound is the most accurate confirmation and will override LMP dating when the discrepancy exceeds ACOG's thresholds. 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.