How conception dates are estimated

A conception date can be estimated from three kinds of pregnancy-dating information:

• Recorded due date: subtract 266 days.
• First day of the last menstrual period: add 14 days.
• Pregnancy weeks and days recorded on a specific date: count back to the implied pregnancy-dating start, then add 14 days.

These methods are different ways of applying the same standard pregnancy calendar. In that calendar, gestational age begins on the first day of the last menstrual period, about two weeks before fertilization in the standard 28-day-cycle model.

The result is an “around” date, not proof that fertilization happened on that day.

In everyday pregnancy questions, conception usually refers to the time around fertilization. The U.S. National Library of Medicine defines fertilization as the fusion of a sperm cell with an ovum, producing a zygote, and lists conception as a related entry term.

Several events are often treated as though they happened on the same date, but they are not interchangeable:

Pregnancy Metrics estimates the calendar date around fertilization. It does not convert these separate events into one confirmed date.

For a naturally conceived pregnancy, fertilization usually is not directly observed. Even when menstrual cycles appear regular, the timing of ovulation and the fertile window can vary. ASRM describes the fertile window as the six-day interval ending on the day of ovulation and notes that its timing can vary considerably, including in people who report regular cycles.

ACOG explains why an intercourse date cannot identify fertilization precisely: sperm may survive in the reproductive tract for about three days and sometimes up to five days, while an egg survives for about 24 hours after ovulation. Pregnancy can therefore result from intercourse several days before ovulation, not only from intercourse on the day fertilization occurred.

A calendar estimate cannot determine:

• which act of intercourse led to the pregnancy;
• the exact day ovulation occurred;
• the moment fertilization occurred;
• when implantation occurred; or
• biological parentage.

When a current estimated due date is already documented in the pregnancy record, the calendar estimate is:

Estimated conception date = recorded due date − 266 days

The 266-day interval represents 38 weeks from the estimated time of fertilization to the estimated due date. It aligns with the standard 280-day gestational calendar, which begins about 14 days earlier.

Worked example

This result does not reveal how the due date was originally established. The recorded due date may reflect the last menstrual period, early ultrasound, assisted reproductive technology records, or a combination of clinical information.

ACOG recommends determining and documenting gestational age and the estimated due date when reliable menstrual information, the first accurate ultrasound, or both are available. Once a due date is established, later changes should be uncommon and documented.

Use the current due date in the pregnancy record. Do not average several old and new due dates.

When the first day of the last menstrual period is the only reliable dating record, the standard estimate is:

Estimated conception date = first day of the last menstrual period + 14 days

Worked example

This method assumes the standard pregnancy calendar: an approximately 28-day menstrual cycle with ovulation near cycle day 14. It does not prove that ovulation or fertilization occurred exactly 14 days after bleeding began.

The estimate may fit less well when:

• the bleeding date is uncertain;
• the bleeding was not a normal menstrual period;
• cycles are irregular;
• ovulation occurred earlier or later than the standard model assumes;
• the pregnancy record now uses a different due date.

Pregnancy Metrics applies the fixed 14-day convention. It does not infer an individual ovulation day from cycle length, symptoms, an app, or a home test.

A pregnancy record may state a gestational age—such as 8 weeks, 4 days—and the date on which that measurement or record was made.

The calculation has three steps:

1. Convert the recorded gestational age into total days.
2. Count backward from the record date to the implied pregnancy-dating start.
3. Add 14 days to estimate the conception date.

The formulas are:

Total gestational days = completed weeks × 7 + additional days

Implied pregnancy-dating start = record date − total gestational days

Estimated conception date = implied pregnancy-dating start + 14 days

Worked example

Record: 8 weeks, 4 days on March 1, 2024

• 8 weeks × 7 days = 56 days
• 56 + 4 = 60 gestational days
• March 1, 2024 − 60 days = January 1, 2024
• January 1, 2024 + 14 days = around January 15, 2024

The year 2024 is a leap year, and the date arithmetic includes February 29 automatically.

Use the weeks, additional days, and record date from the same record. Do not combine weeks from one appointment with the date of another, and do not round 8 weeks, 4 days to 8 weeks.

An intercourse date is not interchangeable with a fertilization date.

ACOG explains that pregnancy can result from intercourse from five days before ovulation until about one day afterward because sperm may survive for several days and the egg remains viable for about 24 hours. ASRM defines the fertile window for counseling as the six-day interval ending on the day of ovulation.

This means that intercourse on one date may lead to fertilization several days later. If intercourse occurred on more than one date within the fertile window, calendar dating cannot identify which act led to the pregnancy.

Do not enter an intercourse date as though it were a confirmed fertilization date. A conception estimate also cannot be used to determine parentage.

Ovulation tracking may provide useful context, but most methods still estimate rather than directly observe ovulation.

ASRM notes that:

• the fertile window can vary even in generally regular cycles;
• calendar methods assume a relationship between cycle length and ovulation that may not hold precisely for an individual cycle;
• calendar apps have important limits in predicting the exact day of ovulation; and
• after a urinary luteinizing hormone surge, ovulation may occur at any time within the following two days.

Cervical mucus, basal body temperature, and ovulation-prediction tests may help identify a likely fertile period, but they do not convert a later pregnancy into a confirmed fertilization timestamp.

A well-supported ovulation date may be useful for estimating a due date, but in a naturally conceived pregnancy it still does not make the exact fertilization date directly observable.

ACOG states that first-trimester ultrasound, through 13 weeks and 6 days, is the most accurate ultrasound method to establish or confirm gestational age.

An ultrasound can help a care team establish or confirm the pregnancy timeline. If the ultrasound-supported due date becomes the current documented due date, that date should be the anchor for later calendar calculations.

An ultrasound does not directly observe the day fertilization occurred. Converting an ultrasound-based gestational age back to a conception estimate still applies the same pregnancy-dating convention and remains approximate.

Use the gestational age or due date as documented in the pregnancy record. Do not try to interpret crown–rump length, fetal measurements, image quality, or redating thresholds with an online calculator.

Pregnancy tests detect human chorionic gonadotropin, or hCG. MedlinePlus explains that the placenta begins producing hCG after a fertilized egg attaches to the uterine wall. Testing too early can produce a false-negative result because hCG may not yet be high enough to detect.

Therefore:

• a positive test date is not the conception date;
• the first faint positive test does not identify the day fertilization occurred;
• the first positive date depends partly on when testing was performed and whether enough hCG was present to detect; and
• a negative early test does not establish that conception had not occurred.

Do not use the date of a positive pregnancy test as an input for estimating conception.

Two estimates may differ because they were built from different records or assumptions.

Common reasons include:

1. The first day of the last menstrual period was uncertain.
2. Ovulation occurred earlier or later than the standard day-14 assumption.
3. Menstrual cycles were irregular.
4. A due date was later confirmed or revised using early ultrasound.
5. One calculation used an old due date and another used the current documented due date.
6. Pregnancy weeks were rounded or copied without the additional days.
7. The pregnancy-weeks record and record date came from different documents.
8. One result used an intercourse date, app prediction, or pregnancy-test date as though it were a fertilization date.

Do not average several conception estimates. When a pregnancy has an established due date, use the current date documented in the pregnancy record as the most consistent calendar anchor.

The three methods can produce the same date when their starting records describe the same pregnancy timeline.

Assisted reproductive technology uses treatment records that are more specific than ordinary last-period assumptions. ACOG recommends assigning the estimated due date from the embryo age and the date of transfer when pregnancy results from ART.

If you know the embryo-transfer date and embryo day, use the IVF Due Date Calculator rather than trying to reconstruct the timeline from a general conception estimate.

If you only have the current IVF-derived due date, a conception calculator can count back 266 days and provide a calendar estimate. It cannot recreate the laboratory fertilization date, retrieval date, embryo culture history, or transfer details.

• Use the Conception Date Calculator when you want one “around” calendar estimate from a recorded due date, reliable last period, or dated pregnancy-weeks record.
• Use the Pregnancy Due Date Calculator when you want to estimate a due date from a last period, well-supported ovulation date, or documented fertilization date.
• Use the Gestational Age Calculator when you already have a due date and want to know the pregnancy weeks and days on a specific date.
• Use the IVF Due Date Calculator when you have a documented Day-3 or Day-5 embryo transfer date.

A conception date estimate cannot determine:

• the exact date or time of fertilization;
• which act of intercourse led to the pregnancy;
• the exact day of ovulation;
• the date of implantation;
• parentage;
• pregnancy location;
• pregnancy viability;
• fetal growth or wellbeing;
• the cause or urgency of symptoms;
• whether a due date should be changed;
• treatment, testing, or delivery timing; or
• the exact day of birth.

It should not be used for legal, forensic, or parentage decisions.

For pregnancy symptoms, uncertainty about dates that may affect care, or questions about the pregnancy record, contact a qualified healthcare professional.

1. ACOG — Methods for Estimating the Due Date

   Supports establishing and documenting gestational age and the estimated due date from menstrual information, early ultrasound, or ART records; identifies first-trimester ultrasound as the most accurate ultrasound dating method; and states that ART pregnancies should use embryo age and transfer date. The document is marked reaffirmed in 2025.

2. ASRM — Optimizing Natural Fertility: A Committee Opinion

   Defines the fertile window as the six-day interval ending on ovulation, explains that fertile-window timing varies even in regular cycles, and describes limits of calendar apps and ovulation-prediction methods.

3. ACOG — Fertility Awareness-Based Methods of Family Planning

   Explains that sperm usually survive for about three days and sometimes up to five days, an egg survives for about 24 hours after ovulation, and pregnancy can result from intercourse from five days before ovulation until about one day afterward.

4. MedlinePlus — Fetal Development

   Explains why gestational age is counted from the first day of the last menstrual period and why the first two counted weeks usually precede conception.

5. MedlinePlus — Pregnancy Test

   Explains that pregnancy tests detect hCG after implantation begins and that testing too early may produce a false-negative result.

6. National Library of Medicine MeSH — Fertilization

   Defines fertilization as fusion of a spermatozoon with an ovum resulting in a zygote and lists conception as a related entry term.

Pregnancy Metrics provides educational calendar estimates only. This page does not confirm fertilization, intercourse, ovulation, implantation, parentage, pregnancy viability, or any diagnosis. Use the current pregnancy record and qualified healthcare guidance for individual care.