Pregnancy dating and timeline guide

Pregnancy dating uses a 40-week calendar. The starting point is usually the first day of the LMP or a start derived from an established EDD or another supported record. Pregnancy age is then the number of complete calendar days since that start, written as completed weeks plus additional days.

A due date is the 40w0d reference point, not a promised birth date. A conception date derived from the pregnancy calendar is an around-date estimate, not confirmation of the exact timing of intercourse, ovulation, fertilization, or implantation.

If none of these records is reliable, do not invent a precise input. Clinical dating may be more useful than a guessed calendar date.

The pregnancy-calendar start is 0w0d. Fourteen calendar days later is 2w0d; 98 days after the start is 14w0d; 196 days after the start is 28w0d; and 280 days after the start is 40w0d, the EDD reference point.

When the starting input is a current EDD, the calendar start is derived by subtracting 280 days. When the input is dated pregnancy age, the start is derived by subtracting the recorded number of weeks and days. These are counting references, not newly reported menstrual periods.

Pregnancy age is written in completed weeks plus a remainder of 0–6 days. For example, 8w4d means 8 completed weeks and 4 additional days, or 60 total gestational days. It does not mean 8.4 decimal weeks.

The count rolls from 8w6d to 9w0d the next day. Calendar months do not have equal lengths, so pregnancy weeks cannot be converted into a single exact month number without choosing a separate convention.

• Standard LMP model: first day of LMP + 280 days.
• Supported cycle adjustment: LMP + 280 days + (average cycle length − 28 days).
• Well-supported ovulation: ovulation date + 266 days.
• Documented fertilization: fertilization date + 266 days.

The cycle-adjusted model shifts the calendar start and EDD together. It is not a claim that every cycle has ovulation exactly 14 days before the next period, and it is not appropriate when the LMP or average cycle is too uncertain.

Subtracting 266 days from an EDD, adding about 14 days to the pregnancy-calendar start, or using dated pregnancy weeks can produce the same calendar conception estimate. Agreement in arithmetic does not prove the exact biological time of fertilization.

Intercourse can occur before ovulation, ovulation timing varies, fertilization follows ovulation, and implantation is later still. The calendar estimate cannot identify a specific intercourse event or establish parentage.

Two records may produce different EDDs because they were recorded at different times, use different starting evidence, contain an entry error, or reflect a later clinical dating decision. Compare the dates and provenance; do not average them.

The current EDD in the pregnancy record, together with guidance from the prenatal or fertility care team, takes priority over an online calculation. A calendar comparison cannot decide whether a discrepancy requires clinical review or redating.

ACOG identifies first-trimester ultrasound, through 13w6d, as the most accurate ultrasound method to establish or confirm gestational age. Once an EDD is established, later changes should be uncommon and documented.

An online tool does not measure crown–rump length, apply gestation-specific ultrasound discrepancy thresholds, interpret a scan, or replace clinical assessment. Pregnancy without an ultrasound that confirms or revises the EDD before 22w0d is described by ACOG as suboptimally dated.

Assisted reproductive technology (ART) dating uses the embryo age and actual transfer date. A Day-5 transfer uses transfer date +261 days; a Day-3 transfer uses transfer date +263 days. Fresh and frozen transfers both require the specific transfer record being dated.

Do not substitute an egg-retrieval, fertilization, freezing, or thaw date for a frozen embryo transfer date. Use the IVF Due Date Calculator only for a documented Day-3 or Day-5 transfer, and use the clinic’s EDD for other embryo ages or unclear records.

For the transfer-day pregnancy age, fresh and frozen record distinctions, and the Day-3 and Day-5 derivation, read How IVF due dates are calculated.

• the exact date of ovulation, fertilization, intercourse, implantation, labor, or birth;
• pregnancy location, viability, fetal growth or wellbeing, parentage, or treatment outcome;
• whether a due date should be changed; or
• what testing, medication, treatment, or delivery decision is appropriate.

1. ACOG — Methods for Estimating the Due Date

   Supports first-trimester ultrasound dating, documentation and later change of an established EDD, ART dating, redating thresholds, and the suboptimally dated boundary.

2. ACOG — How Your Fetus Grows During Pregnancy

   Supports counting gestational age from the first day of the LMP, completed weeks plus additional days, and conventional trimester boundaries.

3. ACOG — When Pregnancy Goes Past Your Due Date

   Supports the 280-day or 40-week pregnancy calendar and the EDD as an estimate used to follow pregnancy progress.

4. Merck Manual Professional — Evaluation of the Obstetric Patient

   Supports LMP plus 280 days and known conception plus 266 days for estimated delivery, along with ultrasound dating context.

5. ASRM — Optimizing Natural Fertility

   Supports variation in the fertile window and limits of calendar, app, and LH-based methods for locating an individual biological event.

6. MedlinePlus — Fetal Development

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

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