LMP-based dating counts from before conception
That is why gestational age is typically about two weeks ahead of embryonic age in standard obstetric dating.
Clinical dating
Pregnancy Due Date Calculator: How Many Weeks Pregnant Am I?
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Results
Ready to calculate
Choose a method and enter your dates to estimate your due date and gestational age.
Choose a method and enter your dates to estimate your due date and gestational age.
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How to use the calculator
- Choose the dating method that matches the best information you have.
- Enter the requested date and any method-specific details.
- Keep the reference date as today to answer “how many weeks pregnant am I?” or choose another date.
- Select Calculate due date. The calculation stays in your browser.
Which method should I choose?
- LMP: use the first day of your last menstrual period if it is known and your cycles are reasonably regular.
- Conception, ovulation, or IUI: use this when the ovulation or insemination date is more certain than the LMP.
- IVF transfer: use the transfer date and select whether a Day 3, Day 5, or Day 6 embryo was transferred.
- Ultrasound: use the scan date and the gestational age stated in the scan report.
- Clinical due date: use an EDD already assigned by your pregnancy care team to work backwards to gestational age.
How due dates and gestational age are calculated
Obstetric gestational age is counted from the first day of the LMP, not from conception. In the standard 28-day model, that starting point is about two weeks before fertilization, so gestational age is usually about two weeks greater than fetal or conception age.
Every method is converted to an LMP-equivalent date. This calculator uses 14 weeks 0 days for the start of the second trimester and 28 weeks 0 days for the start of the third trimester. It shows 37 to 42 weeks as an estimated delivery window for context, not as a prediction of when birth will occur.
How accurate is the estimate?
A due date is an estimate. LMP dating assumes a reliable period date, a 28-day cycle, and ovulation around day 14; changing cycle length assumes ovulation shifts by the same number of days. LMP estimates are less dependable when cycles are irregular, bleeding dates are uncertain, or ovulation timing varies.
When might an ultrasound change the date?
First-trimester ultrasound can establish or confirm gestational age more accurately than uncertain LMP dating. A clinician may change the EDD when the scan and menstrual dates differ enough under clinical guidance. Once your pregnancy care team confirms an EDD, treat the date in your clinical record as authoritative rather than changing it with this calculator.
Formulas and assumptions
Naegele’s rule estimates the due date as the first day of the LMP plus one year, minus three months, plus seven days. The equivalent and less ambiguous date calculation is LMP + 280 days, or LMP + 40 weeks.
LMP EDD = LMP date + 280 days + (cycle length − 28). The cycle length is the usual number of days from one period’s first day to the next; the adjustment assumes ovulation moves with cycle length.Conception/ovulation/IUI EDD = known date + 266 days.Day 3 IVF EDD = transfer date + 263 days.Day 5 IVF EDD = transfer date + 261 days.Day 6 IVF EDD = transfer date + 260 days.Ultrasound EDD = scan date + (280 days − gestational age in days at scan).LMP-equivalent date = clinical EDD − 280 days.
Worked examples
28-day cycle: LMP January 1, 2026 + 280 days = October 8, 2026.
32-day cycle: January 1, 2026 + 280 days + (32 − 28) days = October 12, 2026.
Known conception: January 15, 2026 + 266 days = October 8, 2026.
Day 3 IVF: a January 18, 2026 transfer + 263 days = October 8, 2026.
Day 5 IVF: a January 20, 2026 transfer + 261 days = October 8, 2026.
Ultrasound dating: a scan on March 19, 2026 reporting 11 weeks 0 days gives an LMP-equivalent date of January 1, 2026; January 1 + 280 days = October 8, 2026.
Editorial standards and medical sources
Author: Starlight Tools editorial team
Last editorial review: 17 July 2026
Medical review status: No named clinician has medically reviewed this page. The calculator is informational and should not override a clinician-confirmed EDD.
Methodology: We translated published obstetric dating conventions into transparent calendar-day formulas, checked all methods against the same LMP-equivalent timeline, and tested representative dates and boundary conditions. We do not diagnose pregnancy or recommend changes to clinical care.
Sources
FAQs
How many weeks pregnant am I?
Enter the best dating information you have and choose a reference date. The calculator counts gestational age from an LMP or LMP-equivalent date and reports completed weeks plus days. Your clinician's confirmed dating remains authoritative.
What if I do not know my last period?
Use a known conception or ovulation date, IVF transfer details, an ultrasound date with the gestational age reported at that scan, or an existing clinical due date. If your dates are uncertain, ask a midwife, obstetrician, or other pregnancy care professional about ultrasound dating.
Can I use this with irregular periods?
You can enter your usual cycle length, but the adjustment assumes ovulation shifts by the same number of days as the cycle length. That assumption may not hold for irregular cycles, so ultrasound or clinician-confirmed dating may be more dependable.
Is LMP or conception dating more accurate?
A truly known conception or ovulation date avoids the standard day-14 ovulation assumption. LMP is often easier to recall but can be less dependable with irregular cycles or uncertain bleeding dates. First-trimester ultrasound and assisted-reproduction dates can provide stronger clinical dating evidence.
Can an ultrasound change my due date?
Yes. A clinician may revise an LMP estimate when an early ultrasound differs enough under clinical dating guidance, especially when the LMP is uncertain. Do not change an established clinical due date yourself; use the date documented by your pregnancy care team.
How is an IVF due date calculated?
The embryo's age is included: add 263 days to a Day 3 transfer, 261 days to a Day 5 transfer, or 260 days to a Day 6 transfer. Your fertility clinic's date remains the clinical source of truth.
Why am I four weeks pregnant if conception was two weeks ago?
Obstetric gestational age starts on the first day of the LMP, about two weeks before conception in the standard 28-day model. Fetal or conception age therefore runs about two weeks behind gestational age.
Will I give birth on my due date?
The EDD is an estimate, not a prediction of the birth date. The calculator shows 37 to 42 weeks as an estimated delivery window for context, but individual care and timing vary.
Does carrying twins change the calculated EDD?
No. The dating formula and EDD are calculated the same way, although twin pregnancies often deliver earlier and need individualized monitoring. Follow the dates and plan provided by your pregnancy care team.
5 Facts
Cycle length can move the estimate
A longer or shorter ovulation pattern can shift the expected due date slightly when using LMP.
Timing
Only a minority of births happen exactly on the due date
The due date is a planning estimate, not a precise delivery prediction.
Expectations
Trimester dates are useful for planning
Appointments, leave planning, and personal milestones are often organized around trimester boundaries.
Preparation
IVF dating can be more exact from transfer details
Transfer timing provides a more specific developmental reference than a generic cycle assumption.
Fertility care
Important Note
This calculator is not medical advice and cannot replace ultrasound dating, prenatal assessment, or clinician guidance. For any pregnancy-related decision, use healthcare advice and official medical records as the authoritative source.