Due Date Calculator
Enter your last menstrual period date or conception date to calculate your estimated due date, current gestational week, trimester, and key prenatal milestone dates. Includes cycle length adjustment for non-28-day cycles.
Enter your values above to see the results.
Tips & Notes
- ✓If your first trimester ultrasound gives a due date that differs by more than 7 days from your LMP calculation, your provider will typically use the ultrasound date — it is based on direct fetal measurement and is more accurate.
- ✓Gestational age is always counted from LMP, not from the positive pregnancy test. Most people test positive at 4–5 weeks gestational age, meaning 2–3 weeks have already passed since conception.
- ✓The nuchal translucency scan window (weeks 11–14) closes permanently after 14 weeks — this is one of the most time-sensitive prenatal screenings. Plan your first prenatal appointment early enough to access it.
- ✓Full term is 37–42 weeks, not just the due date. Babies born at 37–38 weeks (early term) have slightly higher rates of breathing difficulties, feeding challenges, and NICU admission than those born at 39–40 weeks.
- ✓For women with very irregular cycles (varying by 7+ days cycle to cycle), LMP-based dating is particularly unreliable. First-trimester ultrasound becomes essential for accurate dating.
Common Mistakes
- ✗Counting pregnancy weeks from the positive pregnancy test — by the time most home tests are positive, you are already 4–6 weeks pregnant by gestational age.
- ✗Treating the due date as a deadline or target rather than an estimate — planning life events (travel, work cutoffs, visitors) tightly around the due date ignores the 2-week normal delivery window on either side.
- ✗Using LMP-based dating without cycle length adjustment when cycles are clearly not 28 days — a woman with a consistent 35-day cycle whose LMP-based date is used unadjusted may have her due date underestimated by 7 days.
- ✗Not correcting gestational age after a first-trimester ultrasound that differs significantly from LMP dates — ultrasound dating is more accurate in the first trimester and should take precedence when the difference is greater than 7 days.
- ✗Confusing gestational age with fetal age — when a provider says "8 weeks pregnant," the embryo is approximately 6 weeks old from conception. The 2-week difference reflects the LMP dating convention.
Due Date Calculator Overview
The due date is more than a single day on the calendar — it is the anchor point for a full 40-week timeline of development, screening, and care. Getting it right from the start helps ensure every appointment, every test, and every milestone happens at the right time.
Due date calculation — Naegele's Rule:
Naegele's Rule (LMP method — clinical standard): Due Date = First day of LMP + 280 days Adjustment for cycle length: Due Date ± (actual cycle length − 28) days Conception date estimate = LMP + (cycle length − 14) days Alternative from conception: Due Date = Conception date + 266 days
EX: LMP = April 5, cycle length = 32 days Standard Naegele (28-day assumed): April 5 + 280 = January 10 Cycle adjustment (32 − 28 = +4 days): January 10 + 4 = January 14 Estimated conception date = April 5 + (32 − 14) = April 5 + 18 = April 23 If today is October 15: gestational age = 27 weeks 3 days → Third trimester begins
Trimester boundaries and milestone dates:
Trimester boundaries (gestational weeks from LMP): First trimester: Weeks 1–12 — organ formation, embryogenesis, highest miscarriage risk Second trimester: Weeks 13–26 — growth phase, fetal movement, anatomy scan Third trimester: Weeks 27–40 — rapid weight gain, lung maturation, birth preparation Full term definitions: Early term 37–38+6, Full term 39–40+6, Late term 41+, Post-term 42+
EX: Due date = January 14 (from example above) Second trimester starts: April 5 + (28 × 12 / 7) ≈ July 5 (week 13 boundary) Actually: LMP + 13 weeks exactly = LMP + 91 days = July 5 Third trimester starts: LMP + 27 weeks = LMP + 189 days = October 11 Full term date (37 weeks): LMP + 259 days = December 21 Post-dates monitoring begins: LMP + 287 days = January 17 (41 weeks)
Key prenatal appointments and screening timeline:
| Gestational week | Milestone / Screening | Why this timing matters |
|---|---|---|
| Week 6–10 | First prenatal visit + dating ultrasound | Confirms heartbeat, dates pregnancy, screens for ectopic |
| Week 11–14 | Nuchal translucency scan (NT) | First-trimester chromosomal screening — window closes at 14 weeks |
| Week 15–20 | Quad screen blood test (optional) | Neural tube and chromosomal abnormality screening |
| Week 18–22 | Anatomy scan (20-week ultrasound) | Detailed fetal structural assessment — sex identifiable |
| Week 24–28 | Glucose tolerance test | Gestational diabetes screening — mandatory timing |
| Week 28 | Rh(D) immunoglobulin (if Rh negative) | Prevents Rh sensitization in Rh-negative mothers |
| Week 35–37 | Group B Strep (GBS) swab | Determines need for IV antibiotics in labor |
| Week 37 | Full term threshold | Delivery no longer considered preterm |
Dating method accuracy comparison:
| Dating method | Accuracy | Best used when |
|---|---|---|
| LMP + Naegele's Rule | ±2 weeks | Regular 28-day cycles, no early ultrasound available |
| LMP + cycle length adjustment | ±10–14 days | Known cycle length significantly different from 28 days |
| First trimester ultrasound (CRL, 8–13 weeks) | ±5–7 days | Most accurate — preferred over LMP when available |
| Second trimester ultrasound (13–20 weeks) | ±10–14 days | When early ultrasound not performed |
| Conception date (known exactly, e.g. IVF) | ±2–3 days | IVF transfers, artificial insemination with known timing |
Only about 5% of babies arrive on their exact due date. The delivery window for a normal pregnancy spans weeks 37–42, and 90% of births occur within two weeks of the estimated date in either direction. First-time mothers statistically deliver 3–5 days after their due date on average; subsequent births tend to occur slightly earlier. These patterns are averages — individual variation is substantial and cannot be predicted for any specific pregnancy. The due date is the clinical anchor point, not a promise or a deadline.
Frequently Asked Questions
Naegele's Rule produces estimates with a range of approximately ±2 weeks for most pregnancies when applied to women with regular 28-day cycles. Research suggests the actual mean gestational duration from LMP is closer to 283–284 days in first pregnancies and 281–282 days in subsequent ones — slightly longer than the 280-day assumption. First-trimester ultrasound (8–13 weeks), which measures crown-rump length against standardized fetal growth charts, improves accuracy to ±5–7 days and is considered the gold standard for gestational dating when available. When LMP and early ultrasound dating differ by more than 7 days, most obstetric guidelines recommend using the ultrasound date.
These terms classify delivery timing relative to developmental readiness. Early term is 37 weeks 0 days through 38 weeks 6 days — the baby can survive but has higher rates of breathing difficulties, feeding challenges, temperature regulation issues, and short-term NICU admission than full-term babies. Full term is 39 weeks 0 days through 40 weeks 6 days — the period when outcomes are generally optimal. Late term is 41 weeks 0 days through 41 weeks 6 days — outcomes remain good but monitoring increases. Post-term is 42 weeks and beyond — placental function may begin declining, increasing the risk of stillbirth, and induction is typically recommended. Preterm is before 37 weeks, with outcomes improving dramatically with each additional week of gestation.
Due dates are updated after ultrasound when the measured size of the fetus does not match the expected size based on LMP dating. The ultrasound measures crown-rump length (CRL) in the first trimester or multiple fetal biometric parameters (head circumference, abdominal circumference, femur length) in the second trimester, and compares these to standardized growth charts. If the measured gestational age differs by more than 7 days (first trimester) or 14 days (second trimester) from the LMP-calculated age, providers typically revise the due date to reflect the ultrasound measurement. This most commonly happens in women with irregular cycles, uncertain LMP dates, or fetal growth that is naturally at the extreme ends of the normal range.
Yes — if you know your conception date precisely (for example, from IVF embryo transfer, confirmed ovulation testing, or a single episode of intercourse during an otherwise abstinent period), the due date is the conception date plus 266 days (38 weeks). IVF transfers provide the most precise conception dates: a Day 5 blastocyst transfer date plus 261 days gives the EDD, or equivalently, subtract 14 days from the transfer date to get the LMP equivalent and add 280 days. Natural conception dating is less precise because even with confirmed ovulation, fertilization can occur 0–24 hours later and implantation 6–10 days after that.
Going past your due date by a few days to a week is statistically normal — delivery at 41 weeks occurs in roughly 10% of pregnancies. Most providers begin more intensive monitoring from 40–41 weeks: non-stress tests (monitoring fetal heart rate response to movement), biophysical profiles (ultrasound assessment of fetal breathing, movement, tone, and fluid levels), and assessment of the cervix for readiness. Research consistently shows that the risk of stillbirth, meconium aspiration, and difficult labor increases after 41–42 weeks. Most guidelines recommend offering induction of labor at 41–42 weeks. If you reach 42 weeks without delivering, induction is typically recommended rather than continued expectant management due to increasing placental dysfunction.
Yes — this calculator adjusts your due date based on cycle length. The standard Naegele calculation assumes ovulation on day 14 of a 28-day cycle. If your cycles are longer, ovulation occurs later, so the due date shifts later by the same number of days. If your cycles are shorter, ovulation is earlier and the due date shifts earlier. For example, a woman with a consistent 35-day cycle would have her due date calculated as LMP + 280 + 7 days = LMP + 287 days, since she ovulates 7 days later than the 28-day assumption. Conversely, someone with a 21-day cycle would get LMP + 280 − 7 = LMP + 273 days. These adjustments improve LMP-based accuracy, but first-trimester ultrasound remains the most reliable dating method for irregular cycles.