JaneenPregnancy Due Date From Conception: How Many Weeks Pregnant Are You?
Quick answer: if you know the conception date accurately, add 266 days, or 38 weeks, to estimate your due date. To convert fetal age into the gestational age used in prenatal care, add about two weeks. In spontaneous pregnancy, the exact conception day is often uncertain, so clinicians use menstrual history and early ultrasound to establish the official date.
How to calculate a pregnancy due date from conception
- Estimated due date = conception date + 266 days.
- Approximate fetal age = days since conception ÷ 7.
- Approximate gestational age = fetal age + two weeks.
For example, if conception was 56 days ago, fetal age is about 8 weeks while the clinical gestational age is about 10 weeks. Adding 266 days to the conception date gives an estimated due date—not a guaranteed delivery day.
| Known information | Calculation | What it estimates |
|---|---|---|
| Conception date | Add 266 days | Estimated due date |
| Days since conception | Divide by 7 | Approximate fetal age |
| Fetal age | Add about 2 weeks | Clinical gestational age |
Fetal age vs. gestational age
Fetal age describes time since conception. Gestational age—the number on your medical record, ultrasound report, and test schedule—usually begins on the first day of your last menstrual period. That is why someone described as 10 weeks pregnant may have conceived around 8 weeks earlier.
Use the gestational age confirmed by your care team when interpreting scans, scheduling tests, or following development milestones. Do not subtract two weeks from every clinical recommendation.
When is the conception date accurate enough to use?
A conception-based estimate is most useful when fertilization or embryo timing is documented through assisted reproductive technology, or when ovulation was monitored closely. In a spontaneous pregnancy, a date of intercourse or a positive ovulation test identifies a possible window but not necessarily the exact fertilization moment.
The American College of Obstetricians and Gynecologists (ACOG) recommends using the embryo age and transfer date for pregnancies resulting from assisted reproductive technology.
Why early ultrasound can change the estimate
ACOG identifies first-trimester ultrasound—through 13 weeks and 6 days—as the most accurate method for establishing or confirming gestational age. Your clinician compares the scan with the last menstrual period and any reliably known fertility-treatment dates.
Once the best obstetric estimate has been documented, later scans normally assess growth rather than repeatedly changing the due date. A difference on a later scan does not automatically mean the original dating was wrong.
Calculate from your last period, conception, ultrasound, or IVF transfer and view your date in Gregorian or Hijri format.
Download Janeen freeWhat if you only know the date of intercourse?
Treat it as a clue, not a confirmed conception date. Use the first day of your last period in a pregnancy due date calculator, then let your care team compare that estimate with an early scan. If your cycles are irregular or the period date is uncertain, say so at your first visit.
Why a due date is still only an estimate
The National Health Service notes that pregnancy normally lasts from 37 to 42 weeks when counted from the first day of the last period. The estimated due date is valuable for planning care, but it is not a promise that labor will begin on that day.
Frequently asked questions
How do I calculate a due date from conception?
Add 266 days to the known conception date. For example, conception on January 1 gives an estimated due date around September 24. Your care team may confirm or adjust the estimate using clinical information and early ultrasound.
Why does the doctor add two weeks?
Clinical pregnancy dating starts on the first day of the last menstrual period, roughly two weeks before conception in a typical cycle. This standardized system helps clinicians time scans, tests, and pregnancy milestones consistently.
Is intercourse date the same as conception date?
Not necessarily. It can suggest a possible window, but it does not confirm the moment of fertilization. Use menstrual history and early ultrasound when conception was not medically documented.
Can every ultrasound change my due date?
Usually not. After the best early estimate is established, ACOG recommends that later changes be rare and clearly documented. Later scans are commonly used to assess fetal growth rather than restart the pregnancy clock.
The bottom line
A pregnancy due date from conception is calculated by adding 266 days. Add about two weeks to fetal age to understand the gestational age used in prenatal care. Use the result for planning, but follow the date established by your clinician for medical decisions.
Medical sources
- ACOG: Methods for Estimating the Due Date — accessed July 30, 2026.
- ACOG: When Pregnancy Goes Past Your Due Date — accessed July 30, 2026.
- NHS: Pregnancy Due Date Calculator — accessed July 30, 2026.