Due Date Calculator — Free Pregnancy Due Date Estimator
Estimate your baby's due date based on the first day of your last menstrual period and your average cycle length. Track your current week, trimester, and key pregnancy milestones.
Key Milestones
Your Due Date
Estimated Due Date
Wednesday, September 9, 2026
Current Week
12w 0d
Trimester
1st
Days Left
196
Summary: Based on your LMP of Wednesday, December 3, 2025 with a 28-day cycle, your estimated due date is Wednesday, September 9, 2026. You are currently 12 weeks and 0 days pregnant (1st trimester). Estimated conception around Wednesday, December 17, 2025. Only about 4% of babies arrive on the exact due date — discuss with your healthcare provider.
How to Use the Due Date Calculator
- Enter the first day of your last menstrual period (LMP): Use the date picker to select or type the date your most recent period started. This is the first day of bleeding, not when your period ended. The calculator uses this date as the foundation for all estimates, as standard pregnancy dating begins from the LMP. The date picker allows selection within the past 300 days, covering the full range of a possible pregnancy.
- Adjust your average cycle length: Enter the typical number of days in your menstrual cycle. The default is 28 days, the average cycle length used in standard pregnancy dating. If your cycles are consistently shorter (for example, 25 days) or longer (for example, 33 days), updating this value will refine your due date estimate. The calculator accepts cycle lengths from 20 to 45 days. For irregular cycles, use the average of your last three cycles.
- Review your pregnancy progress: The results panel instantly shows your estimated due date, current pregnancy week and day, trimester, number of days remaining, completion percentage with a color-coded progress bar, estimated conception date, and trimester milestone dates. All results update instantly as you adjust inputs.
Share these estimates with your healthcare provider, who may adjust the due date based on ultrasound measurements. The estimated due date is just that — an estimate. Only about 4% of babies are born on the exact calculated date, and a normal full-term delivery can happen anywhere from 37 to 42 weeks gestation.
Due Date Formula and Calculation Method
Naegele's Rule (standard 28-day cycle):
Due Date = LMP + 280 days Adjusted for your cycle length:
Due Date = LMP + 280 + (Cycle Length - 28) days Current Week = (Today - LMP) / 7 Estimated Conception = LMP + (Cycle Length - 14) days Variables Explained
- Last Menstrual Period (LMP): The first day of your most recent menstrual bleeding. This is the standard starting point for pregnancy dating because it is a known and observable event. Under this system, you are considered about 2 weeks pregnant at the time of conception, since ovulation and fertilization typically occur around day 14 of a 28-day cycle.
- 280 days (40 weeks): The standard duration of human pregnancy measured from LMP. This includes approximately 2 weeks before conception and 38 weeks of actual fetal development. This figure represents the median gestation length observed across thousands of pregnancies.
- Cycle Length Adjustment: For cycles that differ from 28 days, the due date is shifted accordingly. A longer cycle means ovulation happened later than day 14, so additional days are added. A shorter cycle means earlier ovulation and earlier due date. The adjustment equals the difference between your cycle length and 28.
- Conception Estimate: Estimated as LMP plus the number of days to ovulation (typically cycle length minus 14). Ovulation usually occurs about 14 days before the next period, regardless of cycle length. The conception date is the probable day of fertilization.
- Trimester Boundaries: First trimester spans weeks 1 to 12, second trimester weeks 13 to 27, and third trimester weeks 28 to 40. These divisions align with major developmental milestones and help organize prenatal care schedules.
Step-by-Step Example
Calculate the due date for an LMP of August 1, 2025, with a 30-day cycle:
- Start with the LMP date: August 1, 2025
- Standard gestation: 280 days
- Cycle adjustment: 30 - 28 = +2 days
- Total days: 280 + 2 = 282 days
- Due date: August 1 + 282 days = May 10, 2026
- Conception estimate: August 1 + (30 - 14) = August 17, 2025
- If today is February 16, 2026 (199 days from LMP): Week 28, Day 3
- Trimester: 3rd (week 28 falls in weeks 28-40)
- Days remaining: 282 - 199 = 83 days
- Progress: (199 / 282) x 100 = 70.6% complete
Practical Examples
Example 1: Lisa's Standard Cycle
Lisa, a 30-year-old first-time mother, has regular 28-day cycles. Her LMP was September 10, 2025. She confirmed her pregnancy in mid-October and wants to know her due date and current status as of February 16, 2026.
- Due Date: September 10 + 280 = June 17, 2026
- Conception estimate: around September 24, 2025
- Days since LMP: 159 days
- Current week: 22 weeks, 5 days
- Trimester: 2nd
- Days remaining: 121
- Progress: 56.8%
Lisa is just past the halfway point. Her anatomy scan at week 20 confirmed the due date matched her LMP-based calculation within 3 days. She uses the milestone dates to plan her baby shower for early May and begins maternity leave planning with HR.
Example 2: Megan with a Longer Cycle
Megan, 34, has 33-day cycles on average. Her LMP was July 15, 2025. She wants to understand how her longer cycle affects the due date compared to the standard calculation.
- Standard due date (28-day): July 15 + 280 = April 21, 2026
- Adjusted due date (33-day): July 15 + 285 = April 26, 2026
- Difference: 5 days later
- Conception estimate: July 15 + 19 = August 3, 2025
- As of Feb 16, 2026: Week 30, Day 3 (adjusted)
- Trimester: 3rd
The 5-day adjustment is clinically significant because it affects the timing of prenatal screenings and the interpretation of fetal growth measurements. Megan's 12-week ultrasound confirmed the adjusted date was within 2 days of the ultrasound estimate, validating the cycle-length correction.
Example 3: Sarah Planning Maternity Leave
Sarah, a 37-year-old architect, has a 26-day cycle and an LMP of October 1, 2025. She needs to coordinate project deadlines and maternity leave well in advance.
- Due date: October 1 + 278 = July 6, 2026
- End of 1st trimester: December 31, 2025
- End of 2nd trimester: April 15, 2026
- As of Feb 16, 2026: Week 19, Day 5
- Trimester: 2nd
Sarah uses the trimester milestones to build her timeline. She shares the news at work after the first trimester, starts transitioning projects in April, and plans to begin leave on June 22 — two weeks before her due date. The date calculator helps her count exact business days between milestones for project scheduling.
Example 4: Kayla's IVF Pregnancy
Kayla, 32, conceived through IVF with a day-5 embryo transfer on November 10, 2025. IVF pregnancies calculate the "LMP" as transfer date minus 19 days (for a day-5 transfer), giving an LMP of October 22, 2025.
- Calculated LMP: October 22, 2025
- Due date: October 22 + 280 = July 28, 2026
- As of Feb 16, 2026: Week 16, Day 6
- Trimester: 2nd
- Days remaining: 162
Kayla's fertility clinic provided the same due date, confirming that this calculator works accurately for IVF pregnancies when the correct "calculated LMP" is used. IVF pregnancies have the advantage of a precisely known conception date, making due date estimates particularly accurate.
Pregnancy Milestone Reference Table
| Week | Trimester | Baby Size | Key Milestone |
|---|---|---|---|
| Week 4 | 1st | Poppy seed | Implantation complete, positive pregnancy test possible |
| Week 8 | 1st | Raspberry | All major organs forming, heartbeat detectable on ultrasound |
| Week 12 | 1st | Lime | End of first trimester, miscarriage risk drops significantly |
| Week 16 | 2nd | Avocado | Sex may be visible on ultrasound, first movements possible |
| Week 20 | 2nd | Banana | Anatomy scan, halfway point, movements felt regularly |
| Week 24 | 2nd | Ear of corn | Viability milestone, lungs developing surfactant |
| Week 28 | 3rd | Eggplant | Third trimester begins, eyes open, sleep-wake cycles develop |
| Week 36 | 3rd | Honeydew melon | Lungs nearly mature, baby may drop into pelvis |
| Week 39 | 3rd | Pumpkin | Full term begins, brain and lungs continue maturing |
| Week 40 | 3rd | Watermelon | Due date, average weight 7.5 lbs, ready for delivery |
Tips and Complete Guide to Due Date Estimation
Understanding Due Date Accuracy
A due date is best understood as the center of a delivery window rather than a fixed deadline. About 50% of first-time mothers deliver after their due date, and the normal range for full-term delivery spans from 37 to 42 weeks. Several factors influence when labor actually begins, including genetics, the mother's health, parity (whether this is a first or subsequent pregnancy), and even the time of year. Studies show that first-time mothers deliver on average about 5 days after the due date, while multiparous individuals tend to deliver slightly earlier.
When to Get an Ultrasound Dating Scan
A first-trimester dating ultrasound between 7 and 13 weeks is the gold standard for confirming gestational age. During this scan, the sonographer measures the embryo's crown-rump length (CRL), which correlates closely with gestational age during early development when growth rates are most consistent across pregnancies. This measurement is accurate to within 3 to 5 days. If the ultrasound date and LMP date differ by more than 5 to 7 days, your provider will typically adopt the ultrasound date as the official due date.
Prenatal Care Timeline Based on Your Due Date
Your due date anchors your entire prenatal care schedule. Standard visits occur every 4 weeks through week 28, every 2 weeks from weeks 28 to 36, and weekly from week 36 until delivery. Key screening appointments include first-trimester screening and dating ultrasound (weeks 11 to 13), anatomy scan (weeks 18 to 22), glucose tolerance test for gestational diabetes (weeks 24 to 28), and Group B streptococcus screening (weeks 35 to 37). Our pregnancy calculator offers additional week-by-week development tracking.
Preparing a Birth Plan and Hospital Bag
Use your due date to work backward and set preparation milestones. Most childbirth educators recommend having a birth plan drafted by week 32, a hospital bag packed by week 36, and a car seat installed and inspected by week 37. Pre-register at your hospital or birth center well in advance so paperwork is completed before labor begins. Having key dates mapped out reduces last-minute stress and ensures you are prepared if baby arrives earlier than expected — about 10% of births in the United States are preterm.
Common Mistakes to Avoid
- Using the wrong LMP date: Always use the first day of bleeding, not the last day. Using the end date can shift the due date by 3 to 7 days and affect screening timing.
- Not accounting for cycle length variation: A 35-day cycle means ovulation occurred about a week later than the 28-day assumption, pushing the due date back by a week. Always adjust for your actual cycle length.
- Treating the due date as a guarantee: Only 4% of babies arrive on the exact due date. Plan for a delivery window of 37 to 42 weeks rather than a single date.
- Ignoring ultrasound corrections: If your early ultrasound suggests a different due date than your LMP calculation, follow your provider's recommendation — ultrasound dating is typically more reliable.
- Comparing your timeline to others: Every pregnancy is unique. Belly size, weight gain, symptom timing, and delivery date vary significantly between individuals and between pregnancies for the same person.
Frequently Asked Questions
A due date calculator based on your last menstrual period (LMP) provides a reasonable estimate, but it is not exact. Only about 4% of babies are born on their calculated due date. About 80% are born within two weeks of the estimated date, either before or after. The accuracy depends on how regular your cycles are and how precisely you know your LMP date. A first-trimester ultrasound (performed between 7 and 13 weeks) is typically the most reliable dating method, accurate to within 3 to 5 days. If the ultrasound-based date differs from the LMP-based date by more than 5 to 7 days, your healthcare provider will likely adjust your due date to match the ultrasound.
Naegele's rule is the standard method used to estimate pregnancy due dates. Named after German obstetrician Franz Naegele, the formula adds 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day menstrual cycle with ovulation occurring on day 14. For cycles that are longer or shorter than 28 days, a modified version adjusts by adding or subtracting the difference. For example, for a 32-day cycle, you would add 284 days instead of 280. While widely used in clinical practice for over 200 years, this method is most accurate for individuals with regular, predictable menstrual cycles.
Pregnancy dating starts from the last menstrual period because the LMP is typically a known and observable date, whereas the exact moment of conception is usually uncertain. Sperm can survive in the reproductive tract for up to 5 days, making it impossible to pinpoint the precise day fertilization occurred without medical observation. This convention means you are technically about 2 weeks 'pregnant' at the time of conception and approximately 4 weeks pregnant when you first miss a period. Despite seeming counterintuitive, this system has been the international standard for centuries because it provides a consistent, reliable reference point for healthcare providers worldwide.
Yes, your due date can be adjusted during pregnancy, most commonly after a first-trimester ultrasound. The American College of Obstetricians and Gynecologists (ACOG) recommends using the most accurate method of gestational age estimation to set the due date. If your early ultrasound shows a significant discrepancy from the LMP-based date (more than 5 to 7 days in the first trimester), the ultrasound date typically takes priority. However, once a due date is established and confirmed by early ultrasound, it should not be changed based on later ultrasounds, as measurements become less accurate as pregnancy progresses.
The American College of Obstetricians and Gynecologists defines full-term pregnancy as 39 weeks 0 days through 40 weeks 6 days. This is distinct from early term (37 weeks 0 days through 38 weeks 6 days), late term (41 weeks 0 days through 41 weeks 6 days), and post-term (42 weeks 0 days and beyond). Babies born at 39 to 40 weeks have the best health outcomes overall. While babies born at 37 to 38 weeks are generally healthy, they have slightly higher rates of breathing difficulties, feeding challenges, and NICU admissions compared to full-term babies. Healthcare providers will generally not induce labor before 39 weeks without a medical reason.
If your periods are irregular, an LMP-based calculation may not be reliable. The best options include: getting an early ultrasound between 7 and 13 weeks, which measures the embryo's crown-rump length and is accurate to within 3 to 5 days; tracking ovulation through basal body temperature charts or ovulation predictor kits and adding 266 days to the estimated ovulation date; or if you underwent IVF, calculating from the embryo transfer date. If you have no records or tracking data at all, your healthcare provider can use clinical assessment tools like fundal height measurement in the second and third trimesters to estimate gestational age, though this is less precise than early ultrasound.
Pregnancy is divided into three trimesters, each spanning approximately 13 to 14 weeks. The first trimester (weeks 1 through 12) is a period of rapid development — all major organs begin forming, the heart starts beating around week 6, and the embryo becomes a fetus by week 8. Common symptoms include nausea, fatigue, and breast tenderness. The second trimester (weeks 13 through 27) is often the most comfortable period. The fetus develops recognizable features, begins moving (quickening is usually felt around weeks 16 to 20), and grows rapidly. The third trimester (weeks 28 through 40) involves significant growth and organ maturation, especially the lungs and brain. Common symptoms include back pain, difficulty sleeping, and Braxton-Hicks contractions.
Going past your due date is common, especially for first-time mothers. If your pregnancy reaches 41 weeks, your healthcare provider will likely increase monitoring, which may include non-stress tests and amniotic fluid assessments. Most providers recommend discussing induction of labor between 41 and 42 weeks because the risks of continuing the pregnancy (including reduced placental function and increased stillbirth risk) begin to outweigh the benefits of waiting. The decision to induce is individualized based on factors including cervical readiness, baby's size and position, amniotic fluid levels, and your overall health. Post-term pregnancy (beyond 42 weeks) occurs in about 5% to 10% of pregnancies.
Cycle length directly affects the due date calculation because it changes when ovulation likely occurred. In a standard 28-day cycle, ovulation typically happens around day 14. For a shorter cycle (such as 24 days), ovulation occurs around day 10, meaning conception happened earlier and the due date moves forward by 4 days. For a longer cycle (such as 35 days), ovulation occurs around day 21, meaning conception happened later and the due date is pushed back by 7 days. This is why our calculator adjusts the standard 280-day calculation based on your cycle length. The adjustment formula is: Due Date = LMP + 280 + (Cycle Length - 28) days.
The calculated due date for twins is the same as for a singleton (40 weeks from LMP), but twin pregnancies typically deliver earlier. The average twin delivery occurs at 36 to 37 weeks. The American College of Obstetricians and Gynecologists recommends that uncomplicated dichorionic-diamniotic (di-di) twins be delivered between 38 weeks 0 days and 38 weeks 6 days, while monochorionic-diamniotic twins should be delivered between 36 weeks 0 days and 37 weeks 6 days due to higher complication risks. If you are expecting multiples, work closely with your healthcare provider or maternal-fetal medicine specialist for a personalized delivery timeline rather than relying solely on a standard due date calculator.
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Disclaimer: This calculator is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for medical guidance.
Last updated: February 23, 2026
Sources
- American College of Obstetricians and Gynecologists (ACOG) — Methods for Estimating the Due Date: acog.org/clinical/clinical-guidance
- World Health Organization — Pregnancy, Childbirth, Postpartum and Newborn Care: who.int/publications
- National Institute of Child Health and Human Development — Pregnancy Information: nichd.nih.gov/health/topics/pregnancy