Pregnancy Week Calculator
Calculate completed pregnancy weeks and days from an LMP, clinician-confirmed due date, known conception date, IVF transfer, or clinical ultrasound report. See the week in progress, trimester, estimated due date, current-week dates, and key pregnancy milestones.
- 5 dating methods
- Exact weeks and days
- Trimester and term range
- Calendar-safe calculations
Calculate How Many Weeks Pregnant You Are
Choose the most reliable dating information you have. If your maternity professional has assigned an estimated due date, use that date for consistent week calculations.
Your pregnancy week estimate will appear here.
Progress toward 40 weeks
Waiting for inputCurrent week and key dates
| Date item | Calendar date | Gestational timing | Status on selected date |
|---|---|---|---|
| Calculate a result to view the pregnancy timeline. | |||
Calculation steps
- Select a dating method and enter a valid date.
- The calculator will derive one pregnancy dating start, then count completed weeks and remaining days.
Privacy note: All calculations run on this page. The dates you enter are not sent or stored by this tool.
How to Use This Pregnancy Week Calculator
- Select the dating method that matches your most reliable information.
- For LMP dating, enter the first day of the last menstrual period and the usual regular cycle length. Do not enter the last day of bleeding.
- If your maternity professional has assigned an estimated due date, select the confirmed EDD method and enter that date.
- For known conception or ovulation, use a biologically supported date rather than a date of intercourse.
- For IVF, enter the embryo transfer date and select a day-3 or day-5 embryo.
- For ultrasound dating, copy the completed gestational weeks and additional days printed on the clinical report.
- Leave “Calculate pregnancy week on” set to today, or choose another calendar date.
- Select “Calculate Pregnancy Week.” Review completed weeks and days, the week in progress, trimester, EDD, current-week dates, and calculation steps.
What Does a Pregnancy Week Calculator Show?
A pregnancy week calculator estimates obstetric gestational age on a selected date. The result is written in completed weeks plus additional days. A result of 18 weeks and 4 days means 18 full seven-day weeks have elapsed and four days have passed since the start of the next week. It does not mean 18.4 decimal weeks.
The tool also describes the week currently in progress. At 18 weeks and 4 days, 18 weeks are complete and the pregnancy is in its 19th gestational week. Clinical records normally lead with completed weeks and days because this notation is exact and avoids the ambiguity of phrases such as “week 19.”
Gestational age is not the same as time since conception. Standard pregnancy dating begins from the first day of the last menstrual period, or from an equivalent date derived from a confirmed EDD, IVF record, conception date, or ultrasound report. In a typical 28-day cycle, this starting point is about 14 days before conception.
Pregnancy Week Formula
Every supported method is converted into one LMP-equivalent pregnancy dating start. The calculator then counts whole calendar days from that date to the selected reference date.
Completed weeks = floor(gestational days / 7)
Additional days = gestational days remainder after division by 7
The estimated due date is the dating start plus 280 days. Calendar calculations use UTC date values without clock times, which prevents daylight-saving changes from creating one-day errors.
Last menstrual period method
For a reliable LMP and a regular 28-day cycle, gestational age starts on the first day of the last menstrual period. The standard EDD is 280 days later. For a regular cycle between 21 and 35 days, the tool applies a simple cycle-length adjustment.
Adjusted EDD = LMP + 280 days + (cycle length - 28 days)
This adjustment assumes ovulation shifts by the same number of days as cycle length. It is not reliable when cycles are irregular, the LMP is uncertain, bleeding was not a normal period, or hormonal and postpartum changes affect ovulation.
Confirmed EDD, conception, IVF, and ultrasound methods
- Confirmed EDD: dating start equals the assigned EDD minus 280 days.
- Known conception or ovulation: dating start equals the known date minus 14 days. The EDD is the known date plus 266 days.
- Day-3 IVF transfer: gestational age is 2 weeks and 3 days on transfer day. The EDD is transfer date plus 263 days.
- Day-5 IVF transfer: gestational age is 2 weeks and 5 days on transfer day. The EDD is transfer date plus 261 days.
- Clinical ultrasound report: subtract the reported gestational weeks and days from the scan date to derive the dating start.
The ultrasound option accepts a clinician-reported gestational age. It does not convert crown-rump length, head circumference, femur length, or other fetal measurements into an age. Those measurements require validated clinical equipment and professional interpretation.
Worked Pregnancy Week Example
Suppose the first day of the last menstrual period was May 5, 2026, the regular cycle length was 28 days, and the pregnancy week is being checked on July 28, 2026.
- The cycle adjustment is 28 - 28 = 0 days, so the pregnancy dating start remains May 5.
- There are 84 complete calendar days from May 5 to July 28.
- Divide 84 by 7: 12 complete weeks with 0 additional days.
- The exact gestational age is 12 weeks and 0 days.
- Twelve weeks are complete, so the 13th gestational week is in progress.
- Add 280 days to May 5. The estimated due date is February 9, 2027.
If the same LMP belonged to a regular 30-day cycle, the simple adjustment would move the dating start and EDD forward by two days. A clinician-confirmed EDD should take priority over this consumer estimate.
Comparison of Pregnancy Dating Methods
| Method | Best used when | Dating-start calculation | Main limitation |
|---|---|---|---|
| LMP and cycle length | The first day is certain and cycles are regular | LMP plus cycle length minus 28 days | Assumes predictable ovulation and accurate recall |
| Confirmed EDD | A maternity professional has established the due date | EDD minus 280 days | The entered EDD must match the clinical record |
| Conception or ovulation | The biological date is supported by reliable tracking | Known date minus 14 days | Intercourse date alone does not confirm conception |
| IVF transfer | Pregnancy followed a recorded embryo transfer | Transfer minus 14 days minus embryo age | Use the clinic's ART-derived EDD as the official date |
| Ultrasound report | A clinical report states gestational weeks and days | Scan date minus reported gestational days | Later scans are less accurate for establishing dates |
Pregnancy Weeks, Trimesters, and Term Ranges
This calculator applies day-exact trimester boundaries. The first trimester runs through 13 weeks and 6 days. The second starts at 14 weeks and 0 days. The third starts at 28 weeks and 0 days. It does not convert months into trimester boundaries.
| Stage | Gestational age | Calculator wording |
|---|---|---|
| First trimester | 0 weeks 0 days to 13 weeks 6 days | First trimester |
| Second trimester | 14 weeks 0 days to 27 weeks 6 days | Second trimester |
| Third trimester | 28 weeks 0 days onward | Third trimester |
| Early term | 37 weeks 0 days to 38 weeks 6 days | Third trimester, early term |
| Full term | 39 weeks 0 days to 40 weeks 6 days | Third trimester, full term |
| Late term | 41 weeks 0 days to 41 weeks 6 days | Third trimester, late term |
| Postterm | 42 weeks 0 days or later | 42 weeks or later, verify dates and care plan |
Term labels classify timing at birth. They do not determine fetal health or recommend a delivery date. Monitoring, induction, and delivery decisions require individualized maternity care.
Gestational Age Versus Fetal Age
Gestational age is the standard pregnancy timeline used for clinical appointments and records. It starts from the LMP-equivalent dating date. Fetal or conception age starts around fertilization and is usually about two weeks less than gestational age in the standard model.
For example, a gestational age of 10 weeks generally corresponds to roughly 8 weeks since conception. The difference is approximate in a natural pregnancy because ovulation and fertilization do not always occur on cycle day 14. IVF records provide more direct timing, but obstetric notation still includes the extra two weeks.
Why Pregnancy Months Are Approximate
Pregnancy care uses weeks because calendar months have 28, 29, 30, or 31 days. Four weeks equal 28 days, so ten four-week blocks equal 280 days, but they do not match ten calendar months. Dividing a week number by four produces a rough label, not an exact pregnancy month.
Use completed weeks and days when timing matters. A phrase such as “five months pregnant” is useful in conversation but too imprecise for interpreting a scan report, appointment window, or clinical instruction.
How Accurate Is the Result?
The day count is mathematically exact for the entered dating start, but the dating start itself may be uncertain. LMP dating depends on accurate recall, regular cycles, and an assumption about ovulation. Spotting, irregular cycles, recent hormonal contraception, breastfeeding, a recent pregnancy, or uncertain dates can weaken the estimate.
ACOG guidance identifies first-trimester crown-rump-length ultrasound through 13 weeks and 6 days as the most accurate ultrasound method for establishing or confirming gestational age, commonly within about 5 to 7 days. Accuracy is lower later in pregnancy. A later size difference may reflect fetal growth rather than a wrong EDD, so redating belongs to a clinician.
Once the LMP, early ultrasound, or assisted-reproduction information has established the best obstetric EDD, later week calculations should use that same date. ACOG says subsequent changes should be rare and documented. It considers a pregnancy without an ultrasound confirming or revising the EDD before 22 weeks and 0 days to be suboptimally dated.
Common Pregnancy Week Calculation Mistakes
- Entering the last day of a period instead of its first day.
- Rounding 12 weeks and 6 days up to 13 completed weeks.
- Reading 18 weeks and 4 days as 18.4 decimal weeks.
- Using a pregnancy-test date or intercourse date as the dating start.
- Applying a cycle-length adjustment to irregular cycles.
- Leaving out the additional 0 to 6 days printed on a scan report.
- Selecting day 3 instead of day 5 for an IVF transfer, or the reverse.
- Switching between LMP, scan, and EDD results instead of using one confirmed obstetric date.
- Assuming the EDD is a guaranteed birth date.
Calculation Method and Review Sources
The 280-day LMP estimate, regular-cycle adjustment, and general 37-to-42-week range were reviewed against the NHS pregnancy due-date guidance. Completed-week reporting was checked against the CDC obstetric estimate instructions.
The dating hierarchy, early-ultrasound accuracy, IVF method, and guidance on retaining an established EDD were reviewed against ACOG Committee Opinion 700, hosted by SMFM and its PubMed record. Term ranges were reviewed against NICHD term-pregnancy definitions. Sources inform the educational calculation and do not endorse this website.
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Frequently Asked Questions
How do I calculate how many weeks pregnant I am?
Count calendar days from the LMP-equivalent pregnancy dating start to the selected date. Divide by seven. The whole number is completed weeks, and the remainder is additional days. A clinician-confirmed EDD gives the dating start by counting back 280 days.
Does 12 weeks pregnant mean I am in week 12 or week 13?
At exactly 12 weeks and 0 days, 12 full weeks are complete and the 13th gestational week has started. Clinical notation should remain 12 weeks and 0 days. This calculator shows both labels so the distinction stays clear.
Why does pregnancy dating begin before conception?
Obstetric gestational age traditionally begins on the first day of the last menstrual period. In a typical 28-day cycle, conception occurs about two weeks later. The convention gives maternity care a consistent timeline.
Should I calculate pregnancy weeks from LMP or due date?
Use the best obstetric EDD documented by your maternity professional when one has been established. LMP is useful when the first day is certain and cycles are regular, but early ultrasound or assisted-reproduction dates may provide the clinical anchor.
How does cycle length affect pregnancy weeks?
For a regular 30-day cycle, the simple adjustment moves the dating start and EDD two days later than the standard 28-day estimate. For a regular 26-day cycle, it moves them two days earlier. Do not apply this adjustment to irregular cycles.
How are pregnancy weeks calculated after IVF?
On a day-3 embryo transfer, gestational age is 2 weeks and 3 days. On a day-5 transfer, it is 2 weeks and 5 days. The clinic's ART-derived EDD should remain the official date for future calculations.
Can an ultrasound change my pregnancy week?
An early clinical ultrasound may confirm or revise pregnancy dating when it differs meaningfully from LMP history. Later scans are less accurate for establishing age. A clinician should decide and document any EDD change.
Do twins use a different pregnancy week formula?
No. Basic gestational dating uses the same LMP-equivalent start. Multiple pregnancies often need different monitoring and delivery planning, so follow the dates and care schedule set by the specialist maternity team.
What should I do if the result is 42 weeks or later?
Verify every entered date and contact your maternity team promptly. The calculation cannot assess fetal wellbeing or decide whether monitoring or delivery is needed. Use the clinician-confirmed EDD and care plan.
Health and Calculation Disclaimer
This calculator provides educational date estimates only. It does not confirm pregnancy, fetal age, fetal health, or a delivery date, and it does not replace prenatal care. Use the EDD assigned by your maternity team. Seek urgent medical care for heavy bleeding, fluid leakage, severe or one-sided pain, fainting, chest pain, trouble breathing, severe headache, vision changes, fever of 38°C (100.4°F) or higher, or reduced fetal movement.