Pregnancy dating explained: due dates, weeks and conception

Pregnancy age normally starts with the first day of the last menstrual period (LMP). The conventional pregnancy calendar runs for 40 weeks, or 280 days, even though conception usually occurs later than the calendar start.

An estimated due date (EDD) can be calculated from a reliable LMP, adjusted for average cycle length when appropriate, or from information such as known ovulation, dated pregnancy weeks, early ultrasound, or an embryo transfer. Once a clinical EDD has been established, repeated online calculations should not casually replace it.

Gestational age and age from fertilization are not the same count. LMP dating became useful because the first day of a period can often be identified, while fertilization is usually not observed.

In the standard 28-day model, ovulation and conception are placed around 2 weeks after the LMP. This does not mean conception happened at 0w0d. It means the pregnancy calendar includes roughly 2 weeks before conception so that weeks are counted from one consistent starting point.

Pregnancy age is written as completed weeks plus 0–6 additional days. For example, 8w4d means 8 completed weeks and 4 days, which is 60 gestational days. It does not mean 8.4 decimal weeks.

After 8w6d, the next day is 9w0d. Pregnancy months are less exact because calendar months have different lengths, so there is no single precise week-to-month conversion.

To check pregnancy weeks today or on another date from information you already have, use the Gestational Age Calculator.

The starting information determines the calculation. The standard LMP method adds 280 days; a supported cycle adjustment shifts that estimate by the difference from a 28-day cycle. A well-supported ovulation or documented conception date uses a 266-day interval to the EDD. Dated pregnancy age and IVF transfer records use their own dating paths.

Ovulation does not occur on the same cycle day for everyone or in every cycle. Sperm can remain capable of fertilization for days, so an intercourse date is not the same as a fertilization date. Implantation occurs later and is another separate event.

A calendar can therefore estimate around when conception may have occurred, but it cannot identify the exact day, a specific intercourse event, implantation, or parentage. The Conception Date Calculator keeps that result explicitly approximate.

An LMP estimate can differ from ultrasound dating when the period date is uncertain, the cycle is longer or shorter than the standard model, or ovulation happened earlier or later than the model assumes. Early ultrasound supplies separate dating information based on the pregnancy observed at that exam.

A care team considers the available clinical information when establishing or revising an EDD. An online calculator cannot interpret ultrasound measurements or decide whether a pregnancy should be clinically redated.

If a prenatal or fertility care team has already established a current EDD, use that date for care and pregnancy planning. Do not average two due dates, repeatedly recalculate until one looks preferable, or treat an online estimate as overriding the established clinical date.

Suppose the first day of the LMP was January 1, 2024. The standard pregnancy calendar starts that day at 0w0d. Around January 15 is approximately 2w0d, when conception is placed in the standard model. A pregnancy age of 8w4d is 60 gestational days. The standard EDD is October 7, 2024—280 days after the LMP.

The example shows how the count fits together. It does not prove that ovulation or conception occurred on January 15, and it does not predict the actual birth date.

A simple calendar estimate is less useful when the LMP is uncertain, cycles vary substantially, recent hormonal changes make the usual cycle less representative, fertility treatment supplies a different dating record, or clinical information conflicts with the estimate. Ultrasound dating questions need clinical interpretation rather than another online calculation.

For embryo-transfer dating, read How IVF due dates are calculated or use the IVF Due Date Calculator with a documented Day-3 or Day-5 transfer.

• Pregnancy Due Date Calculator — estimate an EDD from a supported starting date.
• Gestational Age Calculator — count completed weeks and additional days.
• Conception Date Calculator — estimate around when conception may have occurred.
• IVF Due Date Calculator — date a documented Day-3 or Day-5 embryo transfer.

1. ACOG — Methods for Estimating the Due Date

   Supports using early ultrasound and menstrual information to establish an EDD, documenting that date, limiting later changes, and dating pregnancies from assisted reproduction.

2. ACOG — How Your Fetus Grows During Pregnancy

   Supports counting gestational age from the first day of the LMP and expressing pregnancy age in completed weeks plus additional days.

3. ACOG — When Pregnancy Goes Past Your Due Date

   Supports the conventional 40-week pregnancy calendar and explains that an EDD is an estimate rather than a guaranteed delivery day.

4. Merck Manual Professional — Evaluation of the Obstetric Patient

   Supports estimating delivery from LMP plus 280 days or, when conception is known, conception plus 266 days.

5. ASRM — Optimizing Natural Fertility

   Supports variation in fertile-window timing and the limits of calendar, app, and ovulation-marker estimates for locating an individual biological event.

6. MedlinePlus — Fetal Development

   Supports counting gestational age from the first day of the LMP and explains why the first counted weeks usually precede conception.

These sources support the mapped statements above. They do not review, certify, sponsor, or endorse Pregnancy Metrics.