Pregnancy Calculator

Calculate your due date and pregnancy milestones.

Calculation Method

First Day of Last Period

Results

Estimated Due Date

Weeks Pregnant --

Days Pregnant --

First Trimester Ends --

Second Trimester Ends --

About Pregnancy Calculation

Pregnancy is calculated from the first day of your last menstrual period (LMP). Conception typically occurs about 14 days after LMP in a regular 28-day cycle.

A full-term pregnancy is considered 40 weeks (280 days) from LMP. Your healthcare provider will use this date to track your pregnancy progress and schedule prenatal care.

Frequently Asked Questions

How is pregnancy due date calculated?

A pregnancy due date is typically calculated by adding 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle and ovulation on day 14.

What are the trimesters?

First trimester: Weeks 1-13, Second trimester: Weeks 14-27, Third trimester: Weeks 28-40. Each trimester brings different developmental milestones.

Is the due date accurate?

Only about 5% of babies are born on their due date. A due date is an estimate, and most babies arrive within 2 weeks before or after.

A pregnancy calculator turns the date of the last menstrual period (LMP) or a known conception date into the current week of pregnancy, the trimester, and the estimated due date. Obstetric care providers around the world track pregnancy in weeks, not months, because fetal development moves fast and clinical decisions (when to schedule scans, when to screen for gestational diabetes, when delivery is considered 'term') are tied to week ranges. Knowing the current week and the date that each trimester ends helps align personal planning with medical milestones.

The most common input is the first day of the last menstrual period. Pregnancy is dated from that point because ovulation and conception are harder to pin down, while most people can recall when their last period started. From the LMP, the calculator adds 280 days (40 weeks) to get the estimated due date, also called the Estimated Date of Delivery (EDD) or Expected Date of Confinement (EDC). The American College of Obstetricians and Gynecologists (ACOG) and the UK's NHS both use this convention. A small fraction of babies arrive on the EDD, and most arrive within a window of about two weeks before or after.

The tool also reports the current gestational age in weeks and days, the trimester (1st: weeks 1-13, 2nd: weeks 14-27, 3rd: weeks 28-40), and typical developmental notes for that week. For someone who knows their ovulation or conception date, the calculator can back-solve the LMP by subtracting 14 days, since ovulation typically occurs about two weeks after the start of a cycle in a 28-day pattern. Cycles that are shorter or longer shift this midpoint, so cycle length is an optional refinement.

Pregnancy dating is most accurate when an early ultrasound (ideally in the first trimester) is combined with the LMP. If the two disagree by more than about a week, clinicians usually follow the ultrasound estimate, since the LMP can be uncertain. The result here is a useful planning tool, but the dating used for actual medical decisions should always come from a provider.

How to use

Enter the first day of the last menstrual period (LMP), or the conception or ovulation date if it is known precisely.
Add the average cycle length if it is not 28 days; the calculator will adjust the conception window and the due date.
Submit to see the current gestational week and day, the trimester, and the estimated due date.
Compare the result with any ultrasound dating; if they differ by more than a week, follow the provider's adjusted date.

Formula

Estimated Due Date (EDD) = LMP + 280 days (40 weeks), based on Naegele's Rule. For non-28-day cycles, add or subtract the difference in cycle length: EDD = LMP + 280 days + (cycle length − 28).

Interpreting your results

The current week and day show exactly how far along the pregnancy is. Week 1 starts on the first day of the LMP, even though conception typically happens around week 2. The first trimester covers weeks 1 to 13, the second weeks 14 to 27, and the third weeks 28 to 40. A full-term pregnancy is 39 to 40 weeks; delivery between 37 and 38 weeks is 'early term,' and 41+ weeks is 'late term,' with 42 weeks being 'post-term' per ACOG definitions. The CDC and March of Dimes publish week-by-week fetal development guides that pair well with the week number this tool returns.

Frequently asked questions

How accurate is the due date from the last period?

For a regular 28-day cycle, the LMP-based estimate is accurate to within about 1 to 2 weeks. ACOG notes that only about 1 in 20 babies arrive on the EDD, and most are born within a window of about 10 days before or after. An early ultrasound improves accuracy.

What if my cycle is longer or shorter than 28 days?

Cycle length shifts the day of ovulation. The calculator adjusts the due date by adding the difference: a 30-day cycle moves the EDD 2 days later, a 26-day cycle moves it 2 days earlier. Irregular cycles reduce the accuracy of any LMP-based estimate.

When does the first trimester end?

The first trimester covers weeks 1 through 13 (end of week 13), the second trimester covers weeks 14 through 27, and the third trimester covers weeks 28 through delivery. The CDC and NHS use these same week-based boundaries for clinical care.

Can the calculator be used after a positive pregnancy test?

Yes. Once a test is positive, the gestational age is still counted from the LMP. Home tests turn positive around the time of the missed period, roughly 4 weeks after the LMP, and a first prenatal visit typically confirms the dating and adjusts it if needed.