Due Date Calculator

Calculate your pregnancy due date accurately.

What Is a Due Date Calculator?

A due date calculator estimates the date your baby is expected to arrive based on the first day of your last menstrual period. Using the well-established Naegele's rule, the tool adds 280 days to your LMP date, adjusts for your individual cycle length, and provides a comprehensive pregnancy timeline including current gestational age, trimester information, and key milestone dates.

Knowing your due date is essential for scheduling prenatal care, planning leave from work, preparing your home for the baby, and understanding the developmental stage of your pregnancy at any given time. While the due date is an estimate rather than a guarantee, it serves as the central reference point around which all pregnancy care is organized.

How Naegele's Rule Works

Naegele's rule, developed by German obstetrician Franz Naegele in the early 19th century, is the standard method for calculating due dates. The formula adds 280 days to the first day of the last menstrual period, which corresponds to 40 weeks of gestation. This calculation assumes that ovulation and conception occur on day 14 of a 28-day cycle.

For women with cycles other than 28 days, the calculator applies an adjustment. It adds the difference between the actual cycle length and 28 to the standard 280 days. This adjustment accounts for the fact that ovulation occurs later in longer cycles and earlier in shorter cycles, shifting the true conception date accordingly.

The estimated conception date falls approximately 14 days before the adjusted due date minus 266 days, corresponding to the typical ovulation day in the given cycle. The calculator displays this date to provide context about when pregnancy likely began.

How to Use This Calculator

Enter the first day of your last menstrual period. Select the date using the date picker. This should be the first day of actual menstrual bleeding, not spotting.
Enter your average cycle length. The default is 28 days. If you track your cycles and know your average length, enter that value for a more accurate estimate.
Review your results. The calculator shows your estimated due date, current gestational age with trimester, a pregnancy timeline with key milestones, and the expected delivery window.

Worked Examples

Example 1: Standard 28-Day Cycle

A woman whose LMP was October 1, 2025, with a 28-day cycle. Due date = October 1 + 280 days = July 8, 2026. If checked on February 19, 2026, she is at 20 weeks and 0 days, in the second trimester.

Example 2: Longer Cycle

A woman whose LMP was September 15, 2025, with a 33-day cycle. Adjusted due date = September 15 + 285 days = June 27, 2026. The extra 5 days account for later ovulation.

Prenatal Care Timeline

Your due date anchors a series of important prenatal appointments and screenings. During the first trimester, an initial dating ultrasound confirms gestational age and checks for viability. First-trimester screening for chromosomal conditions is offered between weeks 11 and 14.

The second trimester brings the anatomy scan between weeks 18 and 22, which evaluates fetal organs, growth, and the placenta. Glucose screening for gestational diabetes is performed between weeks 24 and 28. Regular visits occur monthly during this period.

Third-trimester care increases in frequency. Visits become biweekly around week 28 and weekly from week 36 until delivery. Group B streptococcus screening occurs between weeks 36 and 37. Fetal position assessment helps plan for delivery approach.

Tips for Using Your Due Date

Share the due date range, not just the specific date. Telling family and friends that the baby is expected in a particular month or week reduces pressure and unsolicited check-ins as the date approaches. Most babies arrive within the two-week window around the due date.

Plan for flexibility in work and travel. Avoid scheduling non-refundable commitments close to your due date. Most airlines restrict travel after 36 weeks of pregnancy. Consider completing major preparations by week 36 in case of early delivery.

Attend all scheduled prenatal appointments. The due date anchors a carefully designed screening and monitoring schedule. Each appointment is timed to detect specific conditions at the gestational age when they first become detectable or treatable.

Keep your LMP date and cycle length records accessible. You will be asked for this information repeatedly throughout pregnancy. Having accurate records prevents confusion, especially if switching providers or hospitals.

Frequently Asked Questions

How is a due date calculated from the last menstrual period?

The due date is calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day menstrual cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the calculator adjusts by adding or subtracting the difference, since ovulation timing shifts with cycle length.

Why is my due date based on my period and not conception?

Medical convention dates pregnancies from the last menstrual period because this date is almost always known with certainty, while the conception date usually is not. Even women who know when intercourse occurred cannot be sure exactly when fertilization happened. The LMP provides a consistent, reliable starting point for all pregnancies regardless of how conception timing was tracked.

Can my due date change during pregnancy?

Yes, due dates may be adjusted after a first-trimester ultrasound. If the ultrasound-measured gestational age differs from the LMP-based estimate by more than seven days, providers typically adopt the ultrasound date. Due dates should not be changed based on ultrasounds after the first trimester because later measurements reflect individual growth variation rather than true gestational age differences.

What percentage of babies are born on their due date?

Only about 4 to 5 percent of babies are born on their exact due date. Most deliveries occur within a two-week window around the due date. Approximately 80 percent of babies are born between weeks 38 and 42. First-time mothers are more likely to deliver after their due date, while subsequent pregnancies may deliver slightly earlier on average.

What is the difference between early term, full term, and late term?

The American College of Obstetricians and Gynecologists defines early term as 37 weeks 0 days through 38 weeks 6 days, full term as 39 weeks 0 days through 40 weeks 6 days, late term as 41 weeks 0 days through 41 weeks 6 days, and post-term as 42 weeks 0 days and beyond. These distinctions matter because outcomes improve with each additional week of gestation up to 39 weeks.

Does cycle length really affect the due date?

Yes, cycle length directly affects ovulation timing and therefore the true conception date. A woman with a 35-day cycle ovulates around day 21 rather than day 14, meaning conception occurs a week later than assumed by a standard 280-day calculation. Without adjusting for cycle length, her due date would be set a week too early, potentially leading to unnecessary concerns about post-term pregnancy.

What happens if I go past my due date?

Going past the due date is common and usually not immediately concerning. Healthcare providers typically begin increased monitoring after 40 weeks, which may include non-stress tests and ultrasounds to check amniotic fluid levels. Most providers recommend discussing induction of labor between 41 and 42 weeks because risks of stillbirth and complications increase modestly in post-term pregnancies.

How accurate is the due date for twins or multiples?

Due dates for twins are calculated the same way as singletons, but the expected delivery date is typically earlier. Twins average delivery around 36 to 37 weeks, and triplets around 32 to 34 weeks. Healthcare providers adjust expectations and monitoring schedules accordingly. The standard 40-week due date serves as a reference point but not a realistic delivery expectation for multiples.