How a pregnancy due date is calculated
The estimated due date (EDD) is the day you are 40 weeks — 280 days — from the first day of your last menstrual period (LMP). That convention comes from Naegele’s rule, and both ACOG and the NHS use 280 days as the average length of pregnancy. This calculator adds exactly 280 calendar days rather than the old “subtract three months, add seven days” shortcut, which can drift by a day or two depending on the lengths of the months involved.
You don’t have to date from your period. If you know your conception date, your IVF transfer date, or a gestational age from an early ultrasound, the calculator will resolve any of those to the same kind of due date — every method ultimately produces one EDD, and everything else (your current gestational age, trimester, and the conception estimate) follows from it.
Gestational age vs fetal age
Gestational age is counted from the first day of your last period, which is about two weeks before you actually conceived. That is why you are considered “2 weeks pregnant” on the day of conception. Fetal (embryonic) age counts from conception itself and is always about 14 days less. The “how many weeks pregnant am I” figure on this page is gestational age, expressed as completed weeks and days (for example, 19w5d means 19 weeks and 5 days).
The four dating methods
Each method adds a fixed number of days to the date you provide:
| Dating method | Due date | Notes |
|---|---|---|
| Last period (LMP) | LMP + 280 days | Cycle-adjusted: + (cycle length − 28). The default Naegele’s-rule method. |
| Conception / ovulation | Conception + 266 days | 266 = 280 − 14, the gestational-vs-fetal two-week offset. |
| IVF — Day 3 (cleavage) | Transfer + 263 days | 266 − 3. Same math for fresh and frozen transfers. |
| IVF — Day 5 (blastocyst) | Transfer + 261 days | 266 − 5. |
| IVF — Day 6 (blastocyst) | Transfer + 260 days | 266 − 6. |
| Ultrasound | Exam date + (280 − reported GA in days) | Back-calculates the EDD from the GA the scan already reported. |
Conception + 266 and LMP + 280 are the same answer for a 28-day cycle, because 266 = 280 − 14. The IVF offsets fall straight out of that: a Day-5 blastocyst was conceived five days before transfer, so its due date is transfer + (266 − 5) = transfer + 261.
Worked example
Generated by the same engine that powers the calculator above, for a last period beginning 15 January 2024 with an average 28-day cycle, viewed as of 1 June 2024.
| Step | Result |
|---|---|
| First day of last period (LMP) | 15 January 2024 |
| Estimated due date = LMP + 280 days | 21 October 2024 |
| Estimated conception = LMP + 14 days | 29 January 2024 |
| Gestational age as of 1 June 2024 | 19w 5d (138 days) |
| Trimester | Trimester 2 |
| Pregnancy elapsed | 49% |
The three trimesters
We use the precise perinatal day-level convention for trimester boundaries, and state the exact cutoffs in your result so the label is never ambiguous near a boundary.
| Trimester | Weeks | Gestational days | What’s happening |
|---|---|---|---|
| First trimester | 0w0d – 13w6d | GA days 0–97 | Conception, implantation and all major organ formation. |
| Second trimester | 14w0d – 27w6d | GA days 98–195 | Often the most comfortable stretch; anatomy scan around 18–22 weeks. |
| Third trimester | 28w0d – birth | GA day 196 onward | Rapid growth; viability ~24w, full term from 37w, due date at 40w. |
Some public-health pages phrase the boundaries loosely (for example “weeks 1–12” for the first trimester); the day-level cutoffs above are what clinical calculators use and what this tool reports.
What “term” means
ACOG splits the weeks around the due date into bands: early term is 37w0d–38w6d, full term is 39w0d–40w6d, late term is 41w0d–41w6d, and post-term is 42w0d and beyond. The older, broader “term” band is simply 37–42 weeks. A baby born before 37 weeks is preterm. Roughly 90% of babies arrive within two weeks either side of the due date, which is why the timeline above extends to 42 weeks rather than stopping at 40.
Typical prenatal appointments and screenings
Once you know your due date, the table below shows when key appointments and screenings are typically scheduled for low-risk pregnancies in the United States, based on ACOG and SMFM guidance. Your provider may adjust timing based on your individual circumstances, health history or risk factors — always follow their schedule, not this one.
| Appointment / screening | Typical window | What it checks |
|---|---|---|
| First prenatal visit | Before 10 weeks | Confirms pregnancy, establishes dating, orders initial labs (blood type, CBC, infectious disease screen). |
| First-trimester screening / NT scan | 11w0d – 13w6d | Nuchal translucency ultrasound ± maternal blood test to screen for chromosomal conditions. Must be done before 14 weeks. |
| Cell-free DNA (cfDNA) screening | From ~10 weeks | Non-invasive prenatal test (NIPT) option; offered alongside or instead of first-trimester combined screening. |
| Anatomy scan (anomaly scan) | 18 – 22 weeks | Detailed fetal survey checking brain, spine, heart and limb anatomy; also confirms placental position. |
| Glucose challenge / gestational diabetes screen | 24 – 28 weeks | Standard one-hour glucose challenge test; high-risk patients may be screened earlier. |
| Group B Streptococcus (GBS) test | 36w0d – 37w6d | Vaginal-rectal swab; result guides antibiotic use in labour. |
| Weekly visits begin | From 36 weeks | Frequency increases from every 2 weeks (28–36w) to every week until delivery. |
Typical windows per ACOG / SMFM 2025 guidance for low-risk singleton pregnancies. Gestational weeks are counted from the first day of the last period. Individual schedules vary — confirm all appointments with your healthcare provider.
Limitations
The LMP method assumes a regular cycle and a known, certain last period; it is less reliable for irregular cycles, recent hormonal contraception, or an uncertain LMP, where clinical guidance prefers a first-trimester ultrasound. This calculator does not measure a scan — for the ultrasound method it takes the gestational age the sonographer already reported and works backward to a due date; it cannot itself convert a crown-rump-length measurement into a gestational age. Viability (~24 weeks), the term thresholds and the milestone dates are informational reference points, not medical advice.
Frequently asked questions
How is my due date calculated?+
Using Naegele’s rule the estimated due date (EDD) is 280 days (40 weeks) after the first day of your last menstrual period. This calculator adds exactly 280 calendar days, and can adjust for cycle length or instead date from a known conception, IVF transfer, or ultrasound.
What is Naegele’s rule?+
Naegele’s rule estimates the due date as the first day of the last period minus three months plus seven days plus one year. For a 28-day cycle that is equivalent to adding 280 days; we use the exact +280-day form so the result doesn’t drift with month lengths.
How accurate is a pregnancy due date?+
An EDD is an estimate, not a guarantee. Only about 5% of babies — roughly 1 in 20 — are born on their exact due date, and about 90% arrive within two weeks either side. Normal term is anywhere from 37 to 42 weeks.
What is the difference between gestational age and fetal age?+
Gestational age is counted from the first day of your last period, so you are already considered “2 weeks pregnant” at conception. Fetal (embryonic) age counts from conception itself and is always about 14 days less than gestational age.
How many weeks pregnant am I?+
Your gestational age is the number of completed weeks and days since the first day of your last period. Enter your dates and the calculator shows your current weeks plus days as of today, along with your trimester and how far along you are as a percentage.
How is the due date calculated for IVF?+
For IVF, conception is the transfer date minus the embryo’s age, then 266 days are added. In practice that means adding 263 days to a Day-3 transfer, 261 days to a Day-5 blastocyst, or 260 days to a Day-6 blastocyst. Fresh and frozen transfers use the same math.
Can I calculate my due date from the conception date?+
Yes. If you know your conception or ovulation date, the due date is that date plus 266 days (about 38 weeks). This is often more precise than the period-based method if your cycle is irregular.
When does each trimester start and end?+
Using the perinatal day-level convention, the first trimester runs from 0w0d through 13w6d, the second from 14w0d through 27w6d, and the third from 28w0d to birth. Some sources phrase the boundary weeks slightly differently; we state the exact cutoffs in the result.
Does cycle length change my due date?+
Yes, if your cycle isn’t 28 days. The LMP method assumes ovulation on day 14; a longer or shorter cycle shifts ovulation, so we adjust the due date by (your cycle length − 28) days when you provide it.
Can my due date change during pregnancy?+
It can. An early ultrasound (up to 13 weeks 6 days) measures the embryo directly and is the most accurate way to date a pregnancy, so your provider may revise an LMP-based due date after your first scan.
What does “full term” and “post-term” mean?+
ACOG defines early term as 37w0d–38w6d, full term as 39w0d–40w6d, late term as 41w0d–41w6d, and post-term as 42w0d and beyond. The older, broader “term” band is simply 37–42 weeks. Babies born before 37 weeks are preterm.
What if I don't know the date of my last period?+
If your last period date is uncertain, an early ultrasound — ideally in the first trimester before 13 weeks 6 days — is the most accurate way to establish your due date. You can also use this calculator's ultrasound method: enter the exam date and the gestational age your sonographer reported, and it will back-calculate a due date from those figures. The conception-date method is another option if you tracked ovulation.
Is this pregnancy calculator a substitute for medical advice?+
No. It is an informational estimate only. Always confirm your due date and gestational age with your doctor or midwife, who can date your pregnancy with an ultrasound and account for your individual circumstances.
Sources
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date (the EDD is 280 days after the first day of the LMP; first-trimester ultrasound is the most accurate dating method).
- NHS — Due date calculator (pregnancy is estimated at 280 days / 40 weeks from the first day of the last period; normal term is 37–42 weeks).
- ACOG Committee Opinion No. 579 — Definition of Term Pregnancy (early, full, late and post-term bands).
- Cleveland Clinic — Pregnancy (Naegele’s rule; gestational age dates from the LMP; about a 5% chance of delivering on the estimated date).
- Perinatology.com — Pregnancy due-date calculator (conception + 266; IVF transfer offsets +263 / +261 / +260; corrected LMP for cycle length).
Formulas and sources last reviewed by the TheCalculatorVault team on 27 June 2026. Due dates and gestational ages are estimates for general information, not medical advice — always consult your healthcare provider.
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