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Pregnancy Due Date Calculator

Estimate pregnancy due date from last menstrual period or conception date, with current week and trimester timeline details.

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Fill in the highlighted fields to see your result.

Calculation method
Use the first day of your last period or the conception date.
Used to estimate the current pregnancy week.

Results update as you type.

Pregnancy dates are estimates for general education only and are not medical advice. Please consult your healthcare provider for personalized guidance.

Pregnancy Due Date Calculator

Pregnancy dating shapes appointment timing, screening windows, and family planning, but the arithmetic can be easy to misunderstand. This pregnancy due date calculator documents exactly how the calculator estimates an expected due date from last menstrual period or conception date, and it flags where the current timeline logic needs caution.

What the calculator estimates

The estimated due date, or EDD, is the calendar date 40 weeks after the first day of the last menstrual period in the standard 280-day method. That convention assumes a typical cycle with ovulation about two weeks after the period begins. When conception date is known or estimated, many references use 266 days from conception, which reaches the same 38 weeks after fertilization.

Due dates are planning estimates, not promises. ACOG emphasizes that accurate dating matters for prenatal testing, growth assessment, and decisions around preterm, term, and postterm pregnancy. Early ultrasound may revise dates when menstrual dates are uncertain or inconsistent with measurements. Use this tool for education and planning, then rely on the date assigned by your clinician. Related tools include the gestational age calculator, pregnancy conception calculator, and reverse due date calculator.

Exact formulas used by calculation

The calculator has two modes. In last menstrual period mode, the entered date is treated as the first day of the LMP:

due date=LMP date+280 days\text{due date} = \text{LMP date} + 280\text{ days}

In conception mode, the entered date is treated as the conception date:

due date=conception date+266 days\text{due date} = \text{conception date} + 266\text{ days}

The calculator also creates an internal start date for timeline outputs. For LMP mode:

start date=LMP date\text{start date} = \text{LMP date}

For conception mode, the current calculation does this:

start date=conception date+14 days\text{start date} = \text{conception date} + 14\text{ days}

Trimester and full-term milestones are then calculated from that start date:

first trimester ends=start date+84 days\text{first trimester ends} = \text{start date} + 84\text{ days}

second trimester ends=start date+182 days\text{second trimester ends} = \text{start date} + 182\text{ days}

full term date=start date+259 days\text{full term date} = \text{start date} + 259\text{ days}

Current week uses the absolute difference between start date and the selected current date, then applies ceiling and floor operations:

current week=⌊⌈∣days between start date and current date∣⌉7⌋\text{current week} = \left\lfloor\frac{\left\lceil\left|\text{days between start date and current date}\right|\right\rceil}{7}\right\rfloor

Because the absolute value is used, a current date before the start date can still produce a positive week number.

Example: estimating a pregnancy due date

If the first day of the last menstrual period is January 1, 2026, LMP mode adds 280 days. The estimated due date is October 8, 2026. The start date is January 1. The first trimester ends 84 days later, March 26, 2026. The second trimester ends 182 days later, July 2, 2026. The full term date is 259 days after January 1, or September 17, 2026. If the current date is February 1, the difference from January 1 is 31 days. Ceiling leaves 31, 31 divided by 7 is 4.42, and floor gives current week 4.

In conception mode, if conception date is January 15, 2026, the due date is October 8, 2026 because 266 days are added. The current method sets the timeline start date to January 29, 2026, which is conception plus 14 days. First trimester end, second trimester end, full term date, and current week are then based on January 29 rather than on LMP-equivalent dating.

Interpretation and date ranges

Clinically, gestational age is usually counted from the first day of the last menstrual period, even though fertilization typically occurs later. ACOG describes term pregnancy as early term from 37 weeks 0 days through 38 weeks 6 days, full term from 39 weeks 0 days through 40 weeks 6 days, late term from 41 weeks 0 days through 41 weeks 6 days, and postterm at 42 weeks 0 days and beyond. This calculator’s “Full term date” is the date at 37 weeks from its internal start date.

Due date calculators cannot evaluate fetal growth, bleeding, pain, contractions, ectopic pregnancy risk, miscarriage risk, or whether ovulation occurred when expected. Assisted reproduction, embryo transfer, irregular cycles, recent contraception, and uncertain bleeding dates can all require different dating methods. The pregnancy conception calculator and reverse due date calculator are planning aids, not substitutes for prenatal records.

Limitations, disclaimer, and common mistakes

This page is educational only and is not medical advice. Consult an obstetric clinician or qualified healthcare professional for pregnancy dating, prenatal care, urgent symptoms, and decisions about testing or delivery timing.

Common mistakes include entering the last day of the period instead of the first day, treating a positive pregnancy test date as conception date, assuming every cycle ovulates on day 14, and using this estimate after a clinician has already assigned an official EDD. Another important limitation is current calculate behavior: in conception mode, the due date formula is conventional, but the timeline start date is conception plus 14 days. That makes trimester endpoints and current week later than typical gestational-age counting would suggest. This text describes the current behavior rather than changing it.

Sources

Frequently asked questions

How does the calculator use last menstrual period?
When last menstrual period is selected, the calculator adds 280 calendar days to the entered date. It also uses that same entered date as the start date for trimester endpoints, full term date, and current week calculation shown in the results.
How does it use conception date?
When conception date is selected, the due date is the conception date plus 266 days. The calculator then sets its timeline start date as conception date plus 14 days, which affects the displayed current week, trimester dates, and full term date.
Can this replace an ultrasound due date?
No. A clinician may use early ultrasound, cycle history, embryo transfer information, physical findings, or other medical details to assign the official estimated due date. Use the provider's date for appointments, screening windows, medication timing, and medical decisions safely.
Why are due dates only estimates?
A due date is based on population averages, not a guaranteed birth date. Cycle length, ovulation timing, uncertain dates, ultrasound measurements, embryo transfer details, and individual pregnancy factors can shift dating. Many births occur before or after the estimated date.
What is shown besides the due date?
The result lists current week, first trimester end, second trimester end, and full term date. These timeline dates come from the calculator's internal start date and should be treated as planning estimates, not clinical instructions or prenatal care milestones.

Related calculators

Gestational Age CalculatorEstimate gestational age in weeks and days, due date, trimester, conception date, milestones, and prenatal timing from LMP, conception, or ultrasound dating.Pregnancy Conception CalculatorEstimate a likely conception date, possible conception range, and due date from an estimated due date, last period, cycle length, or ultrasound dating.Reverse Due Date CalculatorWork backward from a pregnancy due date to estimate last menstrual period, likely conception timing, trimester boundaries, milestones, and appointment windows.Age CalculatorCalculate exact age in years, months, and days from a birth date to any age-at date, plus total months, weeks, days, weekday born, and next birthday.Age Difference CalculatorCalculate the age gap between two ages, birth years, or full birth dates, including years, months, days, and total days.Gardening Frost Date CalculatorPlan planting windows from average last spring frost, first fall frost, USDA hardiness zone, and crop sensitivity, including frost-free days and crop schedules.

Pregnancy Due Date Calculator updated at